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diff --git a/doc/develop/driver-model/design.rst b/doc/develop/driver-model/design.rst new file mode 100644 index 0000000000..4e5cecbab6 --- /dev/null +++ b/doc/develop/driver-model/design.rst @@ -0,0 +1,1016 @@ +.. SPDX-License-Identifier: GPL-2.0+ +.. sectionauthor:: Simon Glass <sjg@chromium.org> + +Design Details +============== + +This README contains high-level information about driver model, a unified +way of declaring and accessing drivers in U-Boot. The original work was done +by: + + * Marek Vasut <marex@denx.de> + * Pavel Herrmann <morpheus.ibis@gmail.com> + * Viktor Křivák <viktor.krivak@gmail.com> + * Tomas Hlavacek <tmshlvck@gmail.com> + +This has been both simplified and extended into the current implementation +by: + + * Simon Glass <sjg@chromium.org> + + +Terminology +----------- + +Uclass + a group of devices which operate in the same way. A uclass provides + a way of accessing individual devices within the group, but always + using the same interface. For example a GPIO uclass provides + operations for get/set value. An I2C uclass may have 10 I2C ports, + 4 with one driver, and 6 with another. + +Driver + some code which talks to a peripheral and presents a higher-level + interface to it. + +Device + an instance of a driver, tied to a particular port or peripheral. + + +How to try it +------------- + +Build U-Boot sandbox and run it:: + + make sandbox_defconfig + make + ./u-boot -d u-boot.dtb + + (type 'reset' to exit U-Boot) + + +There is a uclass called 'demo'. This uclass handles +saying hello, and reporting its status. There are two drivers in this +uclass: + + - simple: Just prints a message for hello, doesn't implement status + - shape: Prints shapes and reports number of characters printed as status + +The demo class is pretty simple, but not trivial. The intention is that it +can be used for testing, so it will implement all driver model features and +provide good code coverage of them. It does have multiple drivers, it +handles parameter data and plat (data which tells the driver how +to operate on a particular platform) and it uses private driver data. + +To try it, see the example session below:: + + =>demo hello 1 + Hello '@' from 07981110: red 4 + =>demo status 2 + Status: 0 + =>demo hello 2 + g + r@ + e@@ + e@@@ + n@@@@ + g@@@@@ + =>demo status 2 + Status: 21 + =>demo hello 4 ^ + y^^^ + e^^^^^ + l^^^^^^^ + l^^^^^^^ + o^^^^^ + w^^^ + =>demo status 4 + Status: 36 + => + + +Running the tests +----------------- + +The intent with driver model is that the core portion has 100% test coverage +in sandbox, and every uclass has its own test. As a move towards this, tests +are provided in test/dm. To run them, try:: + + ./test/py/test.py --bd sandbox --build -k ut_dm -v + +You should see something like this:: + + (venv)$ ./test/py/test.py --bd sandbox --build -k ut_dm -v + +make O=/root/u-boot/build-sandbox -s sandbox_defconfig + +make O=/root/u-boot/build-sandbox -s -j8 + ============================= test session starts ============================== + platform linux2 -- Python 2.7.5, pytest-2.9.0, py-1.4.31, pluggy-0.3.1 -- /root/u-boot/venv/bin/python + cachedir: .cache + rootdir: /root/u-boot, inifile: + collected 199 items + + test/py/tests/test_ut.py::test_ut_dm_init PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_adc_bind] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_adc_multi_channel_conversion] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_adc_multi_channel_shot] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_adc_single_channel_conversion] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_adc_single_channel_shot] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_adc_supply] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_adc_wrong_channel_selection] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_autobind] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_autobind_uclass_pdata_alloc] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_autobind_uclass_pdata_valid] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_autoprobe] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_child_post_bind] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_child_post_bind_uclass] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_child_pre_probe_uclass] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_children] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_children_funcs] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_children_iterators] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_parent_data] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_parent_data_uclass] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_parent_ops] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_parent_platdata] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_bus_parent_platdata_uclass] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_children] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_clk_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_clk_periph] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_device_get_uclass_id] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_eth] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_eth_act] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_eth_alias] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_eth_prime] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_eth_rotate] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_fdt] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_fdt_offset] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_fdt_pre_reloc] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_fdt_uclass_seq] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_gpio] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_gpio_anon] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_gpio_copy] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_gpio_leak] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_gpio_phandles] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_gpio_requestf] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_i2c_bytewise] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_i2c_find] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_i2c_offset] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_i2c_offset_len] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_i2c_probe_empty] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_i2c_read_write] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_i2c_speed] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_leak] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_led_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_led_gpio] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_led_label] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_lifecycle] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_mmc_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_net_retry] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_operations] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_ordering] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_pci_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_pci_busnum] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_pci_swapcase] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_platdata] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_power_pmic_get] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_power_pmic_io] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_power_regulator_autoset] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_power_regulator_autoset_list] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_power_regulator_get] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_power_regulator_set_get_current] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_power_regulator_set_get_enable] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_power_regulator_set_get_mode] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_power_regulator_set_get_voltage] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_pre_reloc] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_ram_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_regmap_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_regmap_syscon] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_remoteproc_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_remove] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_reset_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_reset_walk] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_rtc_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_rtc_dual] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_rtc_reset] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_rtc_set_get] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_spi_find] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_spi_flash] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_spi_xfer] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_syscon_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_syscon_by_driver_data] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_timer_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_uclass] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_uclass_before_ready] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_uclass_devices_find] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_uclass_devices_find_by_name] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_uclass_devices_get] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_uclass_devices_get_by_name] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_usb_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_usb_flash] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_usb_keyb] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_usb_multi] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_usb_remove] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_usb_tree] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_usb_tree_remove] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_usb_tree_reorder] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_base] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_bmp] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_bmp_comp] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_chars] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_context] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_rotation1] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_rotation2] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_rotation3] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_text] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_truetype] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_truetype_bs] PASSED + test/py/tests/test_ut.py::test_ut[ut_dm_video_truetype_scroll] PASSED + + ======================= 84 tests deselected by '-kut_dm' ======================= + ================== 115 passed, 84 deselected in 3.77 seconds =================== + +What is going on? +----------------- + +Let's start at the top. The demo command is in cmd/demo.c. It does +the usual command processing and then: + +.. code-block:: c + + struct udevice *demo_dev; + + ret = uclass_get_device(UCLASS_DEMO, devnum, &demo_dev); + +UCLASS_DEMO means the class of devices which implement 'demo'. Other +classes might be MMC, or GPIO, hashing or serial. The idea is that the +devices in the class all share a particular way of working. The class +presents a unified view of all these devices to U-Boot. + +This function looks up a device for the demo uclass. Given a device +number we can find the device because all devices have registered with +the UCLASS_DEMO uclass. + +The device is automatically activated ready for use by uclass_get_device(). + +Now that we have the device we can do things like: + +.. code-block:: c + + return demo_hello(demo_dev, ch); + +This function is in the demo uclass. It takes care of calling the 'hello' +method of the relevant driver. Bearing in mind that there are two drivers, +this particular device may use one or other of them. + +The code for demo_hello() is in drivers/demo/demo-uclass.c: + +.. code-block:: c + + int demo_hello(struct udevice *dev, int ch) + { + const struct demo_ops *ops = device_get_ops(dev); + + if (!ops->hello) + return -ENOSYS; + + return ops->hello(dev, ch); + } + +As you can see it just calls the relevant driver method. One of these is +in drivers/demo/demo-simple.c: + +.. code-block:: c + + static int simple_hello(struct udevice *dev, int ch) + { + const struct dm_demo_pdata *pdata = dev_get_plat(dev); + + printf("Hello from %08x: %s %d\n", map_to_sysmem(dev), + pdata->colour, pdata->sides); + + return 0; + } + + +So that is a trip from top (command execution) to bottom (driver action) +but it leaves a lot of topics to address. + + +Declaring Drivers +----------------- + +A driver declaration looks something like this (see +drivers/demo/demo-shape.c): + +.. code-block:: c + + static const struct demo_ops shape_ops = { + .hello = shape_hello, + .status = shape_status, + }; + + U_BOOT_DRIVER(demo_shape_drv) = { + .name = "demo_shape_drv", + .id = UCLASS_DEMO, + .ops = &shape_ops, + .priv_data_size = sizeof(struct shape_data), + }; + + +This driver has two methods (hello and status) and requires a bit of +private data (accessible through dev_get_priv(dev) once the driver has +been probed). It is a member of UCLASS_DEMO so will register itself +there. + +In U_BOOT_DRIVER it is also possible to specify special methods for bind +and unbind, and these are called at appropriate times. For many drivers +it is hoped that only 'probe' and 'remove' will be needed. + +The U_BOOT_DRIVER macro creates a data structure accessible from C, +so driver model can find the drivers that are available. + +The methods a device can provide are documented in the device.h header. +Briefly, they are: + + * bind - make the driver model aware of a device (bind it to its driver) + * unbind - make the driver model forget the device + * of_to_plat - convert device tree data to plat - see later + * probe - make a device ready for use + * remove - remove a device so it cannot be used until probed again + +The sequence to get a device to work is bind, of_to_plat (if using +device tree) and probe. + + +Platform Data +------------- + +Note: platform data is the old way of doing things. It is +basically a C structure which is passed to drivers to tell them about +platform-specific settings like the address of its registers, bus +speed, etc. Device tree is now the preferred way of handling this. +Unless you have a good reason not to use device tree (the main one +being you need serial support in SPL and don't have enough SRAM for +the cut-down device tree and libfdt libraries) you should stay away +from platform data. + +Platform data is like Linux platform data, if you are familiar with that. +It provides the board-specific information to start up a device. + +Why is this information not just stored in the device driver itself? The +idea is that the device driver is generic, and can in principle operate on +any board that has that type of device. For example, with modern +highly-complex SoCs it is common for the IP to come from an IP vendor, and +therefore (for example) the MMC controller may be the same on chips from +different vendors. It makes no sense to write independent drivers for the +MMC controller on each vendor's SoC, when they are all almost the same. +Similarly, we may have 6 UARTs in an SoC, all of which are mostly the same, +but lie at different addresses in the address space. + +Using the UART example, we have a single driver and it is instantiated 6 +times by supplying 6 lots of platform data. Each lot of platform data +gives the driver name and a pointer to a structure containing information +about this instance - e.g. the address of the register space. It may be that +one of the UARTS supports RS-485 operation - this can be added as a flag in +the platform data, which is set for this one port and clear for the rest. + +Think of your driver as a generic piece of code which knows how to talk to +a device, but needs to know where it is, any variant/option information and +so on. Platform data provides this link between the generic piece of code +and the specific way it is bound on a particular board. + +Examples of platform data include: + + - The base address of the IP block's register space + - Configuration options, like: + - the SPI polarity and maximum speed for a SPI controller + - the I2C speed to use for an I2C device + - the number of GPIOs available in a GPIO device + +Where does the platform data come from? It is either held in a structure +which is compiled into U-Boot, or it can be parsed from the Device Tree +(see 'Device Tree' below). + +For an example of how it can be compiled in, see demo-pdata.c which +sets up a table of driver names and their associated platform data. +The data can be interpreted by the drivers however they like - it is +basically a communication scheme between the board-specific code and +the generic drivers, which are intended to work on any board. + +Drivers can access their data via dev->info->plat. Here is +the declaration for the platform data, which would normally appear +in the board file. + +.. code-block:: c + + static const struct dm_demo_pdata red_square = { + .colour = "red", + .sides = 4. + }; + + static const struct driver_info info[] = { + { + .name = "demo_shape_drv", + .plat = &red_square, + }, + }; + + demo1 = driver_bind(root, &info[0]); + + +Device Tree +----------- + +While plat is useful, a more flexible way of providing device data is +by using device tree. In U-Boot you should use this where possible. Avoid +sending patches which make use of the U_BOOT_DRVINFO() macro unless strictly +necessary. + +With device tree we replace the above code with the following device tree +fragment: + +.. code-block:: c + + red-square { + compatible = "demo-shape"; + colour = "red"; + sides = <4>; + }; + +This means that instead of having lots of U_BOOT_DRVINFO() declarations in +the board file, we put these in the device tree. This approach allows a lot +more generality, since the same board file can support many types of boards +(e,g. with the same SoC) just by using different device trees. An added +benefit is that the Linux device tree can be used, thus further simplifying +the task of board-bring up either for U-Boot or Linux devs (whoever gets to +the board first!). + +The easiest way to make this work it to add a few members to the driver: + +.. code-block:: c + + .plat_auto = sizeof(struct dm_test_pdata), + .of_to_plat = testfdt_of_to_plat, + +The 'auto' feature allowed space for the plat to be allocated +and zeroed before the driver's of_to_plat() method is called. The +of_to_plat() method, which the driver write supplies, should parse +the device tree node for this device and place it in dev->plat. Thus +when the probe method is called later (to set up the device ready for use) +the platform data will be present. + +Note that both methods are optional. If you provide an of_to_plat +method then it will be called first (during activation). If you provide a +probe method it will be called next. See Driver Lifecycle below for more +details. + +If you don't want to have the plat automatically allocated then you +can leave out plat_auto. In this case you can use malloc +in your of_to_plat (or probe) method to allocate the required memory, +and you should free it in the remove method. + +The driver model tree is intended to mirror that of the device tree. The +root driver is at device tree offset 0 (the root node, '/'), and its +children are the children of the root node. + +In order for a device tree to be valid, the content must be correct with +respect to either device tree specification +(https://www.devicetree.org/specifications/) or the device tree bindings that +are found in the doc/device-tree-bindings directory. When not U-Boot specific +the bindings in this directory tend to come from the Linux Kernel. As such +certain design decisions may have been made already for us in terms of how +specific devices are described and bound. In most circumstances we wish to +retain compatibility without additional changes being made to the device tree +source files. + +Declaring Uclasses +------------------ + +The demo uclass is declared like this: + +.. code-block:: c + + UCLASS_DRIVER(demo) = { + .id = UCLASS_DEMO, + }; + +It is also possible to specify special methods for probe, etc. The uclass +numbering comes from include/dm/uclass-id.h. To add a new uclass, add to the +end of the enum there, then declare your uclass as above. + + +Device Sequence Numbers +----------------------- + +U-Boot numbers devices from 0 in many situations, such as in the command +line for I2C and SPI buses, and the device names for serial ports (serial0, +serial1, ...). Driver model supports this numbering and permits devices +to be locating by their 'sequence'. This numbering uniquely identifies a +device in its uclass, so no two devices within a particular uclass can have +the same sequence number. + +Sequence numbers start from 0 but gaps are permitted. For example, a board +may have I2C buses 1, 4, 5 but no 0, 2 or 3. The choice of how devices are +numbered is up to a particular board, and may be set by the SoC in some +cases. While it might be tempting to automatically renumber the devices +where there are gaps in the sequence, this can lead to confusion and is +not the way that U-Boot works. + +Where a device gets its sequence number is controlled by the DM_SEQ_ALIAS +Kconfig option, which can have a different value in U-Boot proper and SPL. +If this option is not set, aliases are ignored. + +Even if CONFIG_DM_SEQ_ALIAS is enabled, the uclass must still have the +DM_UC_FLAG_SEQ_ALIAS flag set, for its devices to be sequenced by aliases. + +With those options set, devices with an alias (e.g. "serial2") will get that +sequence number (e.g. 2). Other devices get the next available number after all +aliases and all existing numbers. This means that if there is just a single +alias "serial2", unaliased serial devices will be assigned 3 or more, with 0 and +1 being unused. + +If CONFIG_DM_SEQ_ALIAS or DM_UC_FLAG_SEQ_ALIAS are not set, all devices will get +sequence numbers in a simple ordering starting from 0. To find the next number +to allocate, driver model scans through to find the maximum existing number, +then uses the next one. It does not attempt to fill in gaps. + +.. code-block:: none + + aliases { + serial2 = "/serial@22230000"; + }; + +This indicates that in the uclass called "serial", the named node +("/serial@22230000") will be given sequence number 2. Any command or driver +which requests serial device 2 will obtain this device. + +More commonly you can use node references, which expand to the full path: + +.. code-block:: none + + aliases { + serial2 = &serial_2; + }; + ... + serial_2: serial@22230000 { + ... + }; + +The alias resolves to the same string in this case, but this version is +easier to read. + +Device sequence numbers are resolved when a device is bound and the number does +not change for the life of the device. + +There are some situations where the uclass must allocate sequence numbers, +since a strictly increase sequence (with devicetree nodes bound first) is not +suitable. An example of this is the PCI bus. In this case, you can set the +uclass DM_UC_FLAG_NO_AUTO_SEQ flag. With this flag set, only devices with an +alias will be assigned a number by driver model. The rest is left to the uclass +to sort out, e.g. when enumerating the bus. + +Note that changing the sequence number for a device (e.g. in a driver) is not +permitted. If it is felt to be necessary, ask on the mailing list. + +Bus Drivers +----------- + +A common use of driver model is to implement a bus, a device which provides +access to other devices. Example of buses include SPI and I2C. Typically +the bus provides some sort of transport or translation that makes it +possible to talk to the devices on the bus. + +Driver model provides some useful features to help with implementing buses. +Firstly, a bus can request that its children store some 'parent data' which +can be used to keep track of child state. Secondly, the bus can define +methods which are called when a child is probed or removed. This is similar +to the methods the uclass driver provides. Thirdly, per-child platform data +can be provided to specify things like the child's address on the bus. This +persists across child probe()/remove() cycles. + +For consistency and ease of implementation, the bus uclass can specify the +per-child platform data, so that it can be the same for all children of buses +in that uclass. There are also uclass methods which can be called when +children are bound and probed. + +Here an explanation of how a bus fits with a uclass may be useful. Consider +a USB bus with several devices attached to it, each from a different (made +up) uclass:: + + xhci_usb (UCLASS_USB) + eth (UCLASS_ETH) + camera (UCLASS_CAMERA) + flash (UCLASS_FLASH_STORAGE) + +Each of the devices is connected to a different address on the USB bus. +The bus device wants to store this address and some other information such +as the bus speed for each device. + +To achieve this, the bus device can use dev->parent_plat in each of its +three children. This can be auto-allocated if the bus driver (or bus uclass) +has a non-zero value for per_child_plat_auto. If not, then +the bus device or uclass can allocate the space itself before the child +device is probed. + +Also the bus driver can define the child_pre_probe() and child_post_remove() +methods to allow it to do some processing before the child is activated or +after it is deactivated. + +Similarly the bus uclass can define the child_post_bind() method to obtain +the per-child platform data from the device tree and set it up for the child. +The bus uclass can also provide a child_pre_probe() method. Very often it is +the bus uclass that controls these features, since it avoids each driver +having to do the same processing. Of course the driver can still tweak and +override these activities. + +Note that the information that controls this behaviour is in the bus's +driver, not the child's. In fact it is possible that child has no knowledge +that it is connected to a bus. The same child device may even be used on two +different bus types. As an example. the 'flash' device shown above may also +be connected on a SATA bus or standalone with no bus:: + + xhci_usb (UCLASS_USB) + flash (UCLASS_FLASH_STORAGE) - parent data/methods defined by USB bus + + sata (UCLASS_AHCI) + flash (UCLASS_FLASH_STORAGE) - parent data/methods defined by SATA bus + + flash (UCLASS_FLASH_STORAGE) - no parent data/methods (not on a bus) + +Above you can see that the driver for xhci_usb/sata controls the child's +bus methods. In the third example the device is not on a bus, and therefore +will not have these methods at all. Consider the case where the flash +device defines child methods. These would be used for *its* children, and +would be quite separate from the methods defined by the driver for the bus +that the flash device is connetced to. The act of attaching a device to a +parent device which is a bus, causes the device to start behaving like a +bus device, regardless of its own views on the matter. + +The uclass for the device can also contain data private to that uclass. +But note that each device on the bus may be a member of a different +uclass, and this data has nothing to do with the child data for each child +on the bus. It is the bus' uclass that controls the child with respect to +the bus. + + +Driver Lifecycle +---------------- + +Here are the stages that a device goes through in driver model. Note that all +methods mentioned here are optional - e.g. if there is no probe() method for +a device then it will not be called. A simple device may have very few +methods actually defined. + +Bind stage +^^^^^^^^^^ + +U-Boot discovers devices using one of these two methods: + +- Scan the U_BOOT_DRVINFO() definitions. U-Boot looks up the name specified + by each, to find the appropriate U_BOOT_DRIVER() definition. In this case, + there is no path by which driver_data may be provided, but the U_BOOT_DRVINFO() + may provide plat. + +- Scan through the device tree definitions. U-Boot looks at top-level + nodes in the the device tree. It looks at the compatible string in each node + and uses the of_match table of the U_BOOT_DRIVER() structure to find the + right driver for each node. In this case, the of_match table may provide a + driver_data value, but plat cannot be provided until later. + +For each device that is discovered, U-Boot then calls device_bind() to create a +new device, initializes various core fields of the device object such as name, +uclass & driver, initializes any optional fields of the device object that are +applicable such as of_offset, driver_data & plat, and finally calls the +driver's bind() method if one is defined. + +At this point all the devices are known, and bound to their drivers. There +is a 'struct udevice' allocated for all devices. However, nothing has been +activated (except for the root device). Each bound device that was created +from a U_BOOT_DRVINFO() declaration will hold the plat pointer specified +in that declaration. For a bound device created from the device tree, +plat will be NULL, but of_offset will be the offset of the device tree +node that caused the device to be created. The uclass is set correctly for +the device. + +The device's sequence number is assigned, either the requested one or the next +available one (after all aliases are processed) if nothing particular is +requested. + +The device's bind() method is permitted to perform simple actions, but +should not scan the device tree node, not initialise hardware, nor set up +structures or allocate memory. All of these tasks should be left for +the probe() method. + +Note that compared to Linux, U-Boot's driver model has a separate step of +probe/remove which is independent of bind/unbind. This is partly because in +U-Boot it may be expensive to probe devices and we don't want to do it until +they are needed, or perhaps until after relocation. + +Reading ofdata +^^^^^^^^^^^^^^ + +Most devices have data in the device tree which they can read to find out the +base address of hardware registers and parameters relating to driver +operation. This is called 'ofdata' (Open-Firmware data). + +The device's of_to_plat() implemnents allocation and reading of +plat. A parent's ofdata is always read before a child. + +The steps are: + + 1. If priv_auto is non-zero, then the device-private space + is allocated for the device and zeroed. It will be accessible as + dev->priv. The driver can put anything it likes in there, but should use + it for run-time information, not platform data (which should be static + and known before the device is probed). + + 2. If plat_auto is non-zero, then the platform data space + is allocated. This is only useful for device tree operation, since + otherwise you would have to specify the platform data in the + U_BOOT_DRVINFO() declaration. The space is allocated for the device and + zeroed. It will be accessible as dev->plat. + + 3. If the device's uclass specifies a non-zero per_device_auto, + then this space is allocated and zeroed also. It is allocated for and + stored in the device, but it is uclass data. owned by the uclass driver. + It is possible for the device to access it. + + 4. If the device's immediate parent specifies a per_child_auto + then this space is allocated. This is intended for use by the parent + device to keep track of things related to the child. For example a USB + flash stick attached to a USB host controller would likely use this + space. The controller can hold information about the USB state of each + of its children. + + 5. If the driver provides an of_to_plat() method, then this is + called to convert the device tree data into platform data. This should + do various calls like dev_read_u32(dev, ...) to access the node and store + the resulting information into dev->plat. After this point, the device + works the same way whether it was bound using a device tree node or + U_BOOT_DRVINFO() structure. In either case, the platform data is now stored + in the plat structure. Typically you will use the + plat_auto feature to specify the size of the platform data + structure, and U-Boot will automatically allocate and zero it for you before + entry to of_to_plat(). But if not, you can allocate it yourself in + of_to_plat(). Note that it is preferable to do all the device tree + decoding in of_to_plat() rather than in probe(). (Apart from the + ugliness of mixing configuration and run-time data, one day it is possible + that U-Boot will cache platform data for devices which are regularly + de/activated). + + 6. The device is marked 'plat valid'. + +Note that ofdata reading is always done (for a child and all its parents) +before probing starts. Thus devices go through two distinct states when +probing: reading platform data and actually touching the hardware to bring +the device up. + +Having probing separate from ofdata-reading helps deal with of-platdata, where +the probe() method is common to both DT/of-platdata operation, but the +of_to_plat() method is implemented differently. + +Another case has come up where this separate is useful. Generation of ACPI +tables uses the of-platdata but does not want to probe the device. Probing +would cause U-Boot to violate one of its design principles, viz that it +should only probe devices that are used. For ACPI we want to generate a +table for each device, even if U-Boot does not use it. In fact it may not +even be possible to probe the device - e.g. an SD card which is not +present will cause an error on probe, yet we still must tell Linux about +the SD card connector in case it is used while Linux is running. + +It is important that the of_to_plat() method does not actually probe +the device itself. However there are cases where other devices must be probed +in the of_to_plat() method. An example is where a device requires a +GPIO for it to operate. To select a GPIO obviously requires that the GPIO +device is probed. This is OK when used by common, core devices such as GPIO, +clock, interrupts, reset and the like. + +If your device relies on its parent setting up a suitable address space, so +that dev_read_addr() works correctly, then make sure that the parent device +has its setup code in of_to_plat(). If it has it in the probe method, +then you cannot call dev_read_addr() from the child device's +of_to_plat() method. Move it to probe() instead. Buses like PCI can +fall afoul of this rule. + +Activation/probe +^^^^^^^^^^^^^^^^ + +When a device needs to be used, U-Boot activates it, by first reading ofdata +as above and then following these steps (see device_probe()): + + 1. All parent devices are probed. It is not possible to activate a device + unless its predecessors (all the way up to the root device) are activated. + This means (for example) that an I2C driver will require that its bus + be activated. + + 2. The device's probe() method is called. This should do anything that + is required by the device to get it going. This could include checking + that the hardware is actually present, setting up clocks for the + hardware and setting up hardware registers to initial values. The code + in probe() can access: + + - platform data in dev->plat (for configuration) + - private data in dev->priv (for run-time state) + - uclass data in dev->uclass_priv (for things the uclass stores + about this device) + + Note: If you don't use priv_auto then you will need to + allocate the priv space here yourself. The same applies also to + plat_auto. Remember to free them in the remove() method. + + 3. The device is marked 'activated' + + 4. The uclass's post_probe() method is called, if one exists. This may + cause the uclass to do some housekeeping to record the device as + activated and 'known' by the uclass. + +Running stage +^^^^^^^^^^^^^ + +The device is now activated and can be used. From now until it is removed +all of the above structures are accessible. The device appears in the +uclass's list of devices (so if the device is in UCLASS_GPIO it will appear +as a device in the GPIO uclass). This is the 'running' state of the device. + +Removal stage +^^^^^^^^^^^^^ + +When the device is no-longer required, you can call device_remove() to +remove it. This performs the probe steps in reverse: + + 1. The uclass's pre_remove() method is called, if one exists. This may + cause the uclass to do some housekeeping to record the device as + deactivated and no-longer 'known' by the uclass. + + 2. All the device's children are removed. It is not permitted to have + an active child device with a non-active parent. This means that + device_remove() is called for all the children recursively at this point. + + 3. The device's remove() method is called. At this stage nothing has been + deallocated so platform data, private data and the uclass data will all + still be present. This is where the hardware can be shut down. It is + intended that the device be completely inactive at this point, For U-Boot + to be sure that no hardware is running, it should be enough to remove + all devices. + + 4. The device memory is freed (platform data, private data, uclass data, + parent data). + + Note: Because the platform data for a U_BOOT_DRVINFO() is defined with a + static pointer, it is not de-allocated during the remove() method. For + a device instantiated using the device tree data, the platform data will + be dynamically allocated, and thus needs to be deallocated during the + remove() method, either: + + - if the plat_auto is non-zero, the deallocation happens automatically + within the driver model core in the unbind stage; or + + - when plat_auto is 0, both the allocation (in probe() + or preferably of_to_plat()) and the deallocation in remove() + are the responsibility of the driver author. + + 5. The device is marked inactive. Note that it is still bound, so the + device structure itself is not freed at this point. Should the device be + activated again, then the cycle starts again at step 2 above. + +Unbind stage +^^^^^^^^^^^^ + +The device is unbound. This is the step that actually destroys the device. +If a parent has children these will be destroyed first. After this point +the device does not exist and its memory has be deallocated. + + +Special cases for removal +------------------------- + +Some devices need to do clean-up before the OS is called. For example, a USB +driver may want to stop the bus. This can be done in the remove() method. +Some special flags are used to determine whether to remove the device: + + DM_FLAG_OS_PREPARE - indicates that the device needs to get ready for OS + boot. The device will be removed just before the OS is booted + DM_REMOVE_ACTIVE_DMA - indicates that the device uses DMA. This is + effectively the same as DM_FLAG_OS_PREPARE, so the device is removed + before the OS is booted + DM_FLAG_VITAL - indicates that the device is 'vital' to the operation of + other devices. It is possible to remove this device after all regular + devices are removed. This is useful e.g. for a clock, which need to + be active during the device-removal phase. + +The dm_remove_devices_flags() function can be used to remove devices based on +their driver flags. + +Data Structures +--------------- + +Driver model uses a doubly-linked list as the basic data structure. Some +nodes have several lists running through them. Creating a more efficient +data structure might be worthwhile in some rare cases, once we understand +what the bottlenecks are. + + +Changes since v1 +---------------- + +For the record, this implementation uses a very similar approach to the +original patches, but makes at least the following changes: + +- Tried to aggressively remove boilerplate, so that for most drivers there + is little or no 'driver model' code to write. +- Moved some data from code into data structure - e.g. store a pointer to + the driver operations structure in the driver, rather than passing it + to the driver bind function. +- Rename some structures to make them more similar to Linux (struct udevice + instead of struct instance, struct plat, etc.) +- Change the name 'core' to 'uclass', meaning U-Boot class. It seems that + this concept relates to a class of drivers (or a subsystem). We shouldn't + use 'class' since it is a C++ reserved word, so U-Boot class (uclass) seems + better than 'core'. +- Remove 'struct driver_instance' and just use a single 'struct udevice'. + This removes a level of indirection that doesn't seem necessary. +- Built in device tree support, to avoid the need for plat +- Removed the concept of driver relocation, and just make it possible for + the new driver (created after relocation) to access the old driver data. + I feel that relocation is a very special case and will only apply to a few + drivers, many of which can/will just re-init anyway. So the overhead of + dealing with this might not be worth it. +- Implemented a GPIO system, trying to keep it simple + + +Pre-Relocation Support +---------------------- + +For pre-relocation we simply call the driver model init function. Only +drivers marked with DM_FLAG_PRE_RELOC or the device tree 'u-boot,dm-pre-reloc' +property are initialised prior to relocation. This helps to reduce the driver +model overhead. This flag applies to SPL and TPL as well, if device tree is +enabled (CONFIG_OF_CONTROL) there. + +Note when device tree is enabled, the device tree 'u-boot,dm-pre-reloc' +property can provide better control granularity on which device is bound +before relocation. While with DM_FLAG_PRE_RELOC flag of the driver all +devices with the same driver are bound, which requires allocation a large +amount of memory. When device tree is not used, DM_FLAG_PRE_RELOC is the +only way for statically declared devices via U_BOOT_DRVINFO() to be bound +prior to relocation. + +It is possible to limit this to specific relocation steps, by using +the more specialized 'u-boot,dm-spl' and 'u-boot,dm-tpl' flags +in the device tree node. For U-Boot proper you can use 'u-boot,dm-pre-proper' +which means that it will be processed (and a driver bound) in U-Boot proper +prior to relocation, but will not be available in SPL or TPL. + +To reduce the size of SPL and TPL, only the nodes with pre-relocation properties +('u-boot,dm-pre-reloc', 'u-boot,dm-spl' or 'u-boot,dm-tpl') are keept in their +device trees (see README.SPL for details); the remaining nodes are always bound. + +Then post relocation we throw that away and re-init driver model again. +For drivers which require some sort of continuity between pre- and +post-relocation devices, we can provide access to the pre-relocation +device pointers, but this is not currently implemented (the root device +pointer is saved but not made available through the driver model API). + + +SPL Support +----------- + +Driver model can operate in SPL. Its efficient implementation and small code +size provide for a small overhead which is acceptable for all but the most +constrained systems. + +To enable driver model in SPL, define CONFIG_SPL_DM. You might want to +consider the following option also. See the main README for more details. + + - CONFIG_SYS_MALLOC_SIMPLE + - CONFIG_DM_WARN + - CONFIG_DM_DEVICE_REMOVE + - CONFIG_DM_STDIO + + +Enabling Driver Model +--------------------- + +Driver model is being brought into U-Boot gradually. As each subsystems gets +support, a uclass is created and a CONFIG to enable use of driver model for +that subsystem. + +For example CONFIG_DM_SERIAL enables driver model for serial. With that +defined, the old serial support is not enabled, and your serial driver must +conform to driver model. With that undefined, the old serial support is +enabled and driver model is not available for serial. This means that when +you convert a driver, you must either convert all its boards, or provide for +the driver to be compiled both with and without driver model (generally this +is not very hard). + +See the main README for full details of the available driver model CONFIG +options. + + +Things to punt for later +------------------------ + +Uclasses are statically numbered at compile time. It would be possible to +change this to dynamic numbering, but then we would require some sort of +lookup service, perhaps searching by name. This is slightly less efficient +so has been left out for now. One small advantage of dynamic numbering might +be fewer merge conflicts in uclass-id.h. |