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-Remote Processor Messaging (rpmsg) Framework
-
-Note: this document describes the rpmsg bus and how to write rpmsg drivers.
-To learn how to add rpmsg support for new platforms, check out remoteproc.txt
-(also a resident of Documentation/).
-
-1. Introduction
-
-Modern SoCs typically employ heterogeneous remote processor devices in
-asymmetric multiprocessing (AMP) configurations, which may be running
-different instances of operating system, whether it's Linux or any other
-flavor of real-time OS.
-
-OMAP4, for example, has dual Cortex-A9, dual Cortex-M3 and a C64x+ DSP.
-Typically, the dual cortex-A9 is running Linux in a SMP configuration,
-and each of the other three cores (two M3 cores and a DSP) is running
-its own instance of RTOS in an AMP configuration.
-
-Typically AMP remote processors employ dedicated DSP codecs and multimedia
-hardware accelerators, and therefore are often used to offload CPU-intensive
-multimedia tasks from the main application processor.
-
-These remote processors could also be used to control latency-sensitive
-sensors, drive random hardware blocks, or just perform background tasks
-while the main CPU is idling.
-
-Users of those remote processors can either be userland apps (e.g. multimedia
-frameworks talking with remote OMX components) or kernel drivers (controlling
-hardware accessible only by the remote processor, reserving kernel-controlled
-resources on behalf of the remote processor, etc..).
-
-Rpmsg is a virtio-based messaging bus that allows kernel drivers to communicate
-with remote processors available on the system. In turn, drivers could then
-expose appropriate user space interfaces, if needed.
-
-When writing a driver that exposes rpmsg communication to userland, please
-keep in mind that remote processors might have direct access to the
-system's physical memory and other sensitive hardware resources (e.g. on
-OMAP4, remote cores and hardware accelerators may have direct access to the
-physical memory, gpio banks, dma controllers, i2c bus, gptimers, mailbox
-devices, hwspinlocks, etc..). Moreover, those remote processors might be
-running RTOS where every task can access the entire memory/devices exposed
-to the processor. To minimize the risks of rogue (or buggy) userland code
-exploiting remote bugs, and by that taking over the system, it is often
-desired to limit userland to specific rpmsg channels (see definition below)
-it can send messages on, and if possible, minimize how much control
-it has over the content of the messages.
-
-Every rpmsg device is a communication channel with a remote processor (thus
-rpmsg devices are called channels). Channels are identified by a textual name
-and have a local ("source") rpmsg address, and remote ("destination") rpmsg
-address.
-
-When a driver starts listening on a channel, its rx callback is bound with
-a unique rpmsg local address (a 32-bit integer). This way when inbound messages
-arrive, the rpmsg core dispatches them to the appropriate driver according
-to their destination address (this is done by invoking the driver's rx handler
-with the payload of the inbound message).
-
-
-2. User API
-
- int rpmsg_send(struct rpmsg_channel *rpdev, void *data, int len);
- - sends a message across to the remote processor on a given channel.
- The caller should specify the channel, the data it wants to send,
- and its length (in bytes). The message will be sent on the specified
- channel, i.e. its source and destination address fields will be
- set to the channel's src and dst addresses.
-
- In case there are no TX buffers available, the function will block until
- one becomes available (i.e. until the remote processor consumes
- a tx buffer and puts it back on virtio's used descriptor ring),
- or a timeout of 15 seconds elapses. When the latter happens,
- -ERESTARTSYS is returned.
- The function can only be called from a process context (for now).
- Returns 0 on success and an appropriate error value on failure.
-
- int rpmsg_sendto(struct rpmsg_channel *rpdev, void *data, int len, u32 dst);
- - sends a message across to the remote processor on a given channel,
- to a destination address provided by the caller.
- The caller should specify the channel, the data it wants to send,
- its length (in bytes), and an explicit destination address.
- The message will then be sent to the remote processor to which the
- channel belongs, using the channel's src address, and the user-provided
- dst address (thus the channel's dst address will be ignored).
-
- In case there are no TX buffers available, the function will block until
- one becomes available (i.e. until the remote processor consumes
- a tx buffer and puts it back on virtio's used descriptor ring),
- or a timeout of 15 seconds elapses. When the latter happens,
- -ERESTARTSYS is returned.
- The function can only be called from a process context (for now).
- Returns 0 on success and an appropriate error value on failure.
-
- int rpmsg_send_offchannel(struct rpmsg_channel *rpdev, u32 src, u32 dst,
- void *data, int len);
- - sends a message across to the remote processor, using the src and dst
- addresses provided by the user.
- The caller should specify the channel, the data it wants to send,
- its length (in bytes), and explicit source and destination addresses.
- The message will then be sent to the remote processor to which the
- channel belongs, but the channel's src and dst addresses will be
- ignored (and the user-provided addresses will be used instead).
-
- In case there are no TX buffers available, the function will block until
- one becomes available (i.e. until the remote processor consumes
- a tx buffer and puts it back on virtio's used descriptor ring),
- or a timeout of 15 seconds elapses. When the latter happens,
- -ERESTARTSYS is returned.
- The function can only be called from a process context (for now).
- Returns 0 on success and an appropriate error value on failure.
-
- int rpmsg_trysend(struct rpmsg_channel *rpdev, void *data, int len);
- - sends a message across to the remote processor on a given channel.
- The caller should specify the channel, the data it wants to send,
- and its length (in bytes). The message will be sent on the specified
- channel, i.e. its source and destination address fields will be
- set to the channel's src and dst addresses.
-
- In case there are no TX buffers available, the function will immediately
- return -ENOMEM without waiting until one becomes available.
- The function can only be called from a process context (for now).
- Returns 0 on success and an appropriate error value on failure.
-
- int rpmsg_trysendto(struct rpmsg_channel *rpdev, void *data, int len, u32 dst)
- - sends a message across to the remote processor on a given channel,
- to a destination address provided by the user.
- The user should specify the channel, the data it wants to send,
- its length (in bytes), and an explicit destination address.
- The message will then be sent to the remote processor to which the
- channel belongs, using the channel's src address, and the user-provided
- dst address (thus the channel's dst address will be ignored).
-
- In case there are no TX buffers available, the function will immediately
- return -ENOMEM without waiting until one becomes available.
- The function can only be called from a process context (for now).
- Returns 0 on success and an appropriate error value on failure.
-
- int rpmsg_trysend_offchannel(struct rpmsg_channel *rpdev, u32 src, u32 dst,
- void *data, int len);
- - sends a message across to the remote processor, using source and
- destination addresses provided by the user.
- The user should specify the channel, the data it wants to send,
- its length (in bytes), and explicit source and destination addresses.
- The message will then be sent to the remote processor to which the
- channel belongs, but the channel's src and dst addresses will be
- ignored (and the user-provided addresses will be used instead).
-
- In case there are no TX buffers available, the function will immediately
- return -ENOMEM without waiting until one becomes available.
- The function can only be called from a process context (for now).
- Returns 0 on success and an appropriate error value on failure.
-
- struct rpmsg_endpoint *rpmsg_create_ept(struct rpmsg_channel *rpdev,
- void (*cb)(struct rpmsg_channel *, void *, int, void *, u32),
- void *priv, u32 addr);
- - every rpmsg address in the system is bound to an rx callback (so when
- inbound messages arrive, they are dispatched by the rpmsg bus using the
- appropriate callback handler) by means of an rpmsg_endpoint struct.
-
- This function allows drivers to create such an endpoint, and by that,
- bind a callback, and possibly some private data too, to an rpmsg address
- (either one that is known in advance, or one that will be dynamically
- assigned for them).
-
- Simple rpmsg drivers need not call rpmsg_create_ept, because an endpoint
- is already created for them when they are probed by the rpmsg bus
- (using the rx callback they provide when they registered to the rpmsg bus).
-
- So things should just work for simple drivers: they already have an
- endpoint, their rx callback is bound to their rpmsg address, and when
- relevant inbound messages arrive (i.e. messages which their dst address
- equals to the src address of their rpmsg channel), the driver's handler
- is invoked to process it.
-
- That said, more complicated drivers might do need to allocate
- additional rpmsg addresses, and bind them to different rx callbacks.
- To accomplish that, those drivers need to call this function.
- Drivers should provide their channel (so the new endpoint would bind
- to the same remote processor their channel belongs to), an rx callback
- function, an optional private data (which is provided back when the
- rx callback is invoked), and an address they want to bind with the
- callback. If addr is RPMSG_ADDR_ANY, then rpmsg_create_ept will
- dynamically assign them an available rpmsg address (drivers should have
- a very good reason why not to always use RPMSG_ADDR_ANY here).
-
- Returns a pointer to the endpoint on success, or NULL on error.
-
- void rpmsg_destroy_ept(struct rpmsg_endpoint *ept);
- - destroys an existing rpmsg endpoint. user should provide a pointer
- to an rpmsg endpoint that was previously created with rpmsg_create_ept().
-
- int register_rpmsg_driver(struct rpmsg_driver *rpdrv);
- - registers an rpmsg driver with the rpmsg bus. user should provide
- a pointer to an rpmsg_driver struct, which contains the driver's
- ->probe() and ->remove() functions, an rx callback, and an id_table
- specifying the names of the channels this driver is interested to
- be probed with.
-
- void unregister_rpmsg_driver(struct rpmsg_driver *rpdrv);
- - unregisters an rpmsg driver from the rpmsg bus. user should provide
- a pointer to a previously-registered rpmsg_driver struct.
- Returns 0 on success, and an appropriate error value on failure.
-
-
-3. Typical usage
-
-The following is a simple rpmsg driver, that sends an "hello!" message
-on probe(), and whenever it receives an incoming message, it dumps its
-content to the console.
-
-#include <linux/kernel.h>
-#include <linux/module.h>
-#include <linux/rpmsg.h>
-
-static void rpmsg_sample_cb(struct rpmsg_channel *rpdev, void *data, int len,
- void *priv, u32 src)
-{
- print_hex_dump(KERN_INFO, "incoming message:", DUMP_PREFIX_NONE,
- 16, 1, data, len, true);
-}
-
-static int rpmsg_sample_probe(struct rpmsg_channel *rpdev)
-{
- int err;
-
- dev_info(&rpdev->dev, "chnl: 0x%x -> 0x%x\n", rpdev->src, rpdev->dst);
-
- /* send a message on our channel */
- err = rpmsg_send(rpdev, "hello!", 6);
- if (err) {
- pr_err("rpmsg_send failed: %d\n", err);
- return err;
- }
-
- return 0;
-}
-
-static void __devexit rpmsg_sample_remove(struct rpmsg_channel *rpdev)
-{
- dev_info(&rpdev->dev, "rpmsg sample client driver is removed\n");
-}
-
-static struct rpmsg_device_id rpmsg_driver_sample_id_table[] = {
- { .name = "rpmsg-client-sample" },
- { },
-};
-MODULE_DEVICE_TABLE(rpmsg, rpmsg_driver_sample_id_table);
-
-static struct rpmsg_driver rpmsg_sample_client = {
- .drv.name = KBUILD_MODNAME,
- .drv.owner = THIS_MODULE,
- .id_table = rpmsg_driver_sample_id_table,
- .probe = rpmsg_sample_probe,
- .callback = rpmsg_sample_cb,
- .remove = __devexit_p(rpmsg_sample_remove),
-};
-
-static int __init init(void)
-{
- return register_rpmsg_driver(&rpmsg_sample_client);
-}
-module_init(init);
-
-static void __exit fini(void)
-{
- unregister_rpmsg_driver(&rpmsg_sample_client);
-}
-module_exit(fini);
-
-Note: a similar sample which can be built and loaded can be found
-in samples/rpmsg/.
-
-4. Allocations of rpmsg channels:
-
-At this point we only support dynamic allocations of rpmsg channels.
-
-This is possible only with remote processors that have the VIRTIO_RPMSG_F_NS
-virtio device feature set. This feature bit means that the remote
-processor supports dynamic name service announcement messages.
-
-When this feature is enabled, creation of rpmsg devices (i.e. channels)
-is completely dynamic: the remote processor announces the existence of a
-remote rpmsg service by sending a name service message (which contains
-the name and rpmsg addr of the remote service, see struct rpmsg_ns_msg).
-
-This message is then handled by the rpmsg bus, which in turn dynamically
-creates and registers an rpmsg channel (which represents the remote service).
-If/when a relevant rpmsg driver is registered, it will be immediately probed
-by the bus, and can then start sending messages to the remote service.
-
-The plan is also to add static creation of rpmsg channels via the virtio
-config space, but it's not implemented yet.