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-PXA2xx SPI on SSP driver HOWTO
-===================================================
-This a mini howto on the pxa2xx_spi driver. The driver turns a PXA2xx
-synchronous serial port into a SPI master controller
-(see Documentation/spi/spi-summary). The driver has the following features
-
-- Support for any PXA2xx SSP
-- SSP PIO and SSP DMA data transfers.
-- External and Internal (SSPFRM) chip selects.
-- Per slave device (chip) configuration.
-- Full suspend, freeze, resume support.
-
-The driver is built around a "spi_message" fifo serviced by workqueue and a
-tasklet. The workqueue, "pump_messages", drives message fifo and the tasklet
-(pump_transfer) is responsible for queuing SPI transactions and setting up and
-launching the dma/interrupt driven transfers.
-
-Declaring PXA2xx Master Controllers
------------------------------------
-Typically a SPI master is defined in the arch/.../mach-*/board-*.c as a
-"platform device". The master configuration is passed to the driver via a table
-found in include/linux/spi/pxa2xx_spi.h:
-
-struct pxa2xx_spi_master {
- u32 clock_enable;
- u16 num_chipselect;
- u8 enable_dma;
-};
-
-The "pxa2xx_spi_master.clock_enable" field is used to enable/disable the
-corresponding SSP peripheral block in the "Clock Enable Register (CKEN"). See
-the "PXA2xx Developer Manual" section "Clocks and Power Management".
-
-The "pxa2xx_spi_master.num_chipselect" field is used to determine the number of
-slave device (chips) attached to this SPI master.
-
-The "pxa2xx_spi_master.enable_dma" field informs the driver that SSP DMA should
-be used. This caused the driver to acquire two DMA channels: rx_channel and
-tx_channel. The rx_channel has a higher DMA service priority the tx_channel.
-See the "PXA2xx Developer Manual" section "DMA Controller".
-
-NSSP MASTER SAMPLE
-------------------
-Below is a sample configuration using the PXA255 NSSP.
-
-static struct resource pxa_spi_nssp_resources[] = {
- [0] = {
- .start = __PREG(SSCR0_P(2)), /* Start address of NSSP */
- .end = __PREG(SSCR0_P(2)) + 0x2c, /* Range of registers */
- .flags = IORESOURCE_MEM,
- },
- [1] = {
- .start = IRQ_NSSP, /* NSSP IRQ */
- .end = IRQ_NSSP,
- .flags = IORESOURCE_IRQ,
- },
-};
-
-static struct pxa2xx_spi_master pxa_nssp_master_info = {
- .clock_enable = CKEN_NSSP, /* NSSP Peripheral clock */
- .num_chipselect = 1, /* Matches the number of chips attached to NSSP */
- .enable_dma = 1, /* Enables NSSP DMA */
-};
-
-static struct platform_device pxa_spi_nssp = {
- .name = "pxa2xx-spi", /* MUST BE THIS VALUE, so device match driver */
- .id = 2, /* Bus number, MUST MATCH SSP number 1..n */
- .resource = pxa_spi_nssp_resources,
- .num_resources = ARRAY_SIZE(pxa_spi_nssp_resources),
- .dev = {
- .platform_data = &pxa_nssp_master_info, /* Passed to driver */
- },
-};
-
-static struct platform_device *devices[] __initdata = {
- &pxa_spi_nssp,
-};
-
-static void __init board_init(void)
-{
- (void)platform_add_device(devices, ARRAY_SIZE(devices));
-}
-
-Declaring Slave Devices
------------------------
-Typically each SPI slave (chip) is defined in the arch/.../mach-*/board-*.c
-using the "spi_board_info" structure found in "linux/spi/spi.h". See
-"Documentation/spi/spi-summary" for additional information.
-
-Each slave device attached to the PXA must provide slave specific configuration
-information via the structure "pxa2xx_spi_chip" found in
-"include/linux/spi/pxa2xx_spi.h". The pxa2xx_spi master controller driver
-will uses the configuration whenever the driver communicates with the slave
-device. All fields are optional.
-
-struct pxa2xx_spi_chip {
- u8 tx_threshold;
- u8 rx_threshold;
- u8 dma_burst_size;
- u32 timeout;
- u8 enable_loopback;
- void (*cs_control)(u32 command);
-};
-
-The "pxa2xx_spi_chip.tx_threshold" and "pxa2xx_spi_chip.rx_threshold" fields are
-used to configure the SSP hardware fifo. These fields are critical to the
-performance of pxa2xx_spi driver and misconfiguration will result in rx
-fifo overruns (especially in PIO mode transfers). Good default values are
-
- .tx_threshold = 8,
- .rx_threshold = 8,
-
-The range is 1 to 16 where zero indicates "use default".
-
-The "pxa2xx_spi_chip.dma_burst_size" field is used to configure PXA2xx DMA
-engine and is related the "spi_device.bits_per_word" field. Read and understand
-the PXA2xx "Developer Manual" sections on the DMA controller and SSP Controllers
-to determine the correct value. An SSP configured for byte-wide transfers would
-use a value of 8. The driver will determine a reasonable default if
-dma_burst_size == 0.
-
-The "pxa2xx_spi_chip.timeout" fields is used to efficiently handle
-trailing bytes in the SSP receiver fifo. The correct value for this field is
-dependent on the SPI bus speed ("spi_board_info.max_speed_hz") and the specific
-slave device. Please note that the PXA2xx SSP 1 does not support trailing byte
-timeouts and must busy-wait any trailing bytes.
-
-The "pxa2xx_spi_chip.enable_loopback" field is used to place the SSP porting
-into internal loopback mode. In this mode the SSP controller internally
-connects the SSPTX pin to the SSPRX pin. This is useful for initial setup
-testing.
-
-The "pxa2xx_spi_chip.cs_control" field is used to point to a board specific
-function for asserting/deasserting a slave device chip select. If the field is
-NULL, the pxa2xx_spi master controller driver assumes that the SSP port is
-configured to use SSPFRM instead.
-
-NOTE: the SPI driver cannot control the chip select if SSPFRM is used, so the
-chipselect is dropped after each spi_transfer. Most devices need chip select
-asserted around the complete message. Use SSPFRM as a GPIO (through cs_control)
-to accommodate these chips.
-
-
-NSSP SLAVE SAMPLE
------------------
-The pxa2xx_spi_chip structure is passed to the pxa2xx_spi driver in the
-"spi_board_info.controller_data" field. Below is a sample configuration using
-the PXA255 NSSP.
-
-/* Chip Select control for the CS8415A SPI slave device */
-static void cs8415a_cs_control(u32 command)
-{
- if (command & PXA2XX_CS_ASSERT)
- GPCR(2) = GPIO_bit(2);
- else
- GPSR(2) = GPIO_bit(2);
-}
-
-/* Chip Select control for the CS8405A SPI slave device */
-static void cs8405a_cs_control(u32 command)
-{
- if (command & PXA2XX_CS_ASSERT)
- GPCR(3) = GPIO_bit(3);
- else
- GPSR(3) = GPIO_bit(3);
-}
-
-static struct pxa2xx_spi_chip cs8415a_chip_info = {
- .tx_threshold = 8, /* SSP hardward FIFO threshold */
- .rx_threshold = 8, /* SSP hardward FIFO threshold */
- .dma_burst_size = 8, /* Byte wide transfers used so 8 byte bursts */
- .timeout = 235, /* See Intel documentation */
- .cs_control = cs8415a_cs_control, /* Use external chip select */
-};
-
-static struct pxa2xx_spi_chip cs8405a_chip_info = {
- .tx_threshold = 8, /* SSP hardward FIFO threshold */
- .rx_threshold = 8, /* SSP hardward FIFO threshold */
- .dma_burst_size = 8, /* Byte wide transfers used so 8 byte bursts */
- .timeout = 235, /* See Intel documentation */
- .cs_control = cs8405a_cs_control, /* Use external chip select */
-};
-
-static struct spi_board_info streetracer_spi_board_info[] __initdata = {
- {
- .modalias = "cs8415a", /* Name of spi_driver for this device */
- .max_speed_hz = 3686400, /* Run SSP as fast a possbile */
- .bus_num = 2, /* Framework bus number */
- .chip_select = 0, /* Framework chip select */
- .platform_data = NULL; /* No spi_driver specific config */
- .controller_data = &cs8415a_chip_info, /* Master chip config */
- .irq = STREETRACER_APCI_IRQ, /* Slave device interrupt */
- },
- {
- .modalias = "cs8405a", /* Name of spi_driver for this device */
- .max_speed_hz = 3686400, /* Run SSP as fast a possbile */
- .bus_num = 2, /* Framework bus number */
- .chip_select = 1, /* Framework chip select */
- .controller_data = &cs8405a_chip_info, /* Master chip config */
- .irq = STREETRACER_APCI_IRQ, /* Slave device interrupt */
- },
-};
-
-static void __init streetracer_init(void)
-{
- spi_register_board_info(streetracer_spi_board_info,
- ARRAY_SIZE(streetracer_spi_board_info));
-}
-
-
-DMA and PIO I/O Support
------------------------
-The pxa2xx_spi driver supports both DMA and interrupt driven PIO message
-transfers. The driver defaults to PIO mode and DMA transfers must be enabled
-by setting the "enable_dma" flag in the "pxa2xx_spi_master" structure. The DMA
-mode supports both coherent and stream based DMA mappings.
-
-The following logic is used to determine the type of I/O to be used on
-a per "spi_transfer" basis:
-
-if !enable_dma then
- always use PIO transfers
-
-if spi_message.len > 8191 then
- print "rate limited" warning
- use PIO transfers
-
-if spi_message.is_dma_mapped and rx_dma_buf != 0 and tx_dma_buf != 0 then
- use coherent DMA mode
-
-if rx_buf and tx_buf are aligned on 8 byte boundary then
- use streaming DMA mode
-
-otherwise
- use PIO transfer
-
-THANKS TO
----------
-
-David Brownell and others for mentoring the development of this driver.
-