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diff --git a/Documentation/spi/pxa2xx.rst b/Documentation/spi/pxa2xx.rst new file mode 100644 index 000000000..882d3cc72 --- /dev/null +++ b/Documentation/spi/pxa2xx.rst @@ -0,0 +1,240 @@ +============================== +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.rst). 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_controller { + u16 num_chipselect; + u8 enable_dma; + }; + +The "pxa2xx_spi_controller.num_chipselect" field is used to determine the number of +slave device (chips) attached to this SPI master. + +The "pxa2xx_spi_controller.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_controller pxa_nssp_master_info = { + .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.rst" 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_controller" 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. |