diff options
Diffstat (limited to 'Documentation/spi')
| -rw-r--r-- | Documentation/spi/pxa2xx.rst | 208 |
1 files changed, 0 insertions, 208 deletions
diff --git a/Documentation/spi/pxa2xx.rst b/Documentation/spi/pxa2xx.rst deleted file mode 100644 index 33d2ad95901e..000000000000 --- a/Documentation/spi/pxa2xx.rst +++ /dev/null @@ -1,208 +0,0 @@ -============================== -PXA2xx SPI on SSP driver HOWTO -============================== - -This a mini HOWTO on the pxa2xx_spi driver. The driver turns a PXA2xx -synchronous serial port into an SPI host controller -(see Documentation/spi/spi-summary.rst). The driver has the following features - -- Support for any PXA2xx and compatible SSP. -- SSP PIO and SSP DMA data transfers. -- External and Internal (SSPFRM) chip selects. -- Per peripheral device (chip) configuration. -- Full suspend, freeze, resume support. - -The driver is built around a &struct spi_message FIFO serviced by kernel -thread. The kernel thread, spi_pump_messages(), drives message FIFO and -is responsible for queuing SPI transactions and setting up and launching -the DMA or interrupt driven transfers. - -Declaring PXA2xx host controllers ---------------------------------- -Typically, for a legacy platform, an SPI host controller is defined in the -arch/.../mach-*/board-*.c as a "platform device". The host controller configuration -is passed to the driver via a table found in include/linux/spi/pxa2xx_spi.h:: - - struct pxa2xx_spi_controller { - u8 num_chipselect; - u8 enable_dma; - ... - }; - -The "pxa2xx_spi_controller.num_chipselect" field is used to determine the number of -peripheral devices (chips) attached to this SPI host controller. - -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 than the Tx channel. -See the "PXA2xx Developer Manual" section "DMA Controller". - -For the new platforms the description of the controller and peripheral devices -comes from Device Tree or ACPI. - -NSSP HOST SAMPLE ----------------- -Below is a sample configuration using the PXA255 NSSP for a legacy platform:: - - 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_controller_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_controller_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 peripheral devices ----------------------------- -Typically, for a legacy platform, each SPI peripheral device (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 peripheral device (chip) attached to the PXA2xx must provide specific chip configuration -information via the structure "pxa2xx_spi_chip" found in -"include/linux/spi/pxa2xx_spi.h". The PXA2xx host controller driver will use -the configuration whenever the driver communicates with the peripheral -device. All fields are optional. - -:: - - struct pxa2xx_spi_chip { - u8 tx_threshold; - u8 rx_threshold; - u8 dma_burst_size; - u32 timeout; - }; - -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 -peripheral device. Please note that the PXA2xx SSP 1 does not support trailing byte -timeouts and must busy-wait any trailing bytes. - -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 a descriptor) -to accommodate these chips. - - -NSSP PERIPHERAL SAMPLE ----------------------- -For a legacy platform or in some other cases, 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. - -:: - - static struct pxa2xx_spi_chip cs8415a_chip_info = { - .tx_threshold = 8, /* SSP hardware FIFO threshold */ - .rx_threshold = 8, /* SSP hardware FIFO threshold */ - .dma_burst_size = 8, /* Byte wide transfers used so 8 byte bursts */ - .timeout = 235, /* See Intel documentation */ - }; - - static struct pxa2xx_spi_chip cs8405a_chip_info = { - .tx_threshold = 8, /* SSP hardware FIFO threshold */ - .rx_threshold = 8, /* SSP hardware FIFO threshold */ - .dma_burst_size = 8, /* Byte wide transfers used so 8 byte bursts */ - .timeout = 235, /* See Intel documentation */ - }; - - 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 possible */ - .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, /* Host controller config */ - .irq = STREETRACER_APCI_IRQ, /* Peripheral device interrupt */ - }, - { - .modalias = "cs8405a", /* Name of spi_driver for this device */ - .max_speed_hz = 3686400, /* Run SSP as fast a possible */ - .bus_num = 2, /* Framework bus number */ - .chip_select = 1, /* Framework chip select */ - .controller_data = &cs8405a_chip_info, /* Host controller config */ - .irq = STREETRACER_APCI_IRQ, /* Peripheral 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. -For the newer platforms, that are known to support DMA, the driver will enable -it automatically and try it first with a possible fallback to PIO. 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 spi_message.len > 65536 then - print "rate limited" warning - use PIO transfers - - if enable_dma and the size is in the range [DMA burst size..65536] then - use streaming DMA mode - - otherwise - use PIO transfer - -THANKS TO ---------- -David Brownell and others for mentoring the development of this driver. |