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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/asm-arm/arch-pxa/pxa2xx_spi.h:
+
+struct pxa2xx_spi_master {
+        enum pxa_ssp_type ssp_type;
+        u32 clock_enable;
+        u16 num_chipselect;
+        u8 enable_dma;
+};
+
+The "pxa2xx_spi_master.ssp_type" field must have a value between 1 and 3 and
+informs the driver which features a particular SSP supports.
+
+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 = {
+        .ssp_type = PXA25x_NSSP, /* Type of SSP */
+        .clock_enable = CKEN9_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/asm-arm/arch-pxa/pxa2xx_spi.h".  The pxa2xx_spi master controller driver
+will uses the configuration whenever the driver communicates with the slave
+device.
+
+struct pxa2xx_spi_chip {
+        u8 tx_threshold;
+        u8 rx_threshold;
+        u8 dma_burst_size;
+        u32 timeout_microsecs;
+        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 = 12,
+        .rx_threshold = 4,
+
+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 "pxa2xx_spi_chip.timeout_microsecs" 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 the 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 the 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.
+
+NSSP SALVE 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 = 12, /* SSP hardward FIFO threshold */
+        .rx_threshold = 4, /* SSP hardward FIFO threshold */
+        .dma_burst_size = 8, /* Byte wide transfers used so 8 byte bursts */
+        .timeout_microsecs = 64, /* Wait at least 64usec to handle trailing */
+        .cs_control = cs8415a_cs_control, /* Use external chip select */
+};
+
+static struct pxa2xx_spi_chip cs8405a_chip_info = {
+        .tx_threshold = 12, /* SSP hardward FIFO threshold */
+        .rx_threshold = 4, /* SSP hardward FIFO threshold */
+        .dma_burst_size = 8, /* Byte wide transfers used so 8 byte bursts */
+        .timeout_microsecs = 64, /* Wait at least 64usec to handle trailing */
+        .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 support both DMA and interrupt driven PIO message
+transfers.  The driver defaults to PIO mode and DMA transfers must enabled by
+setting the "enable_dma" flag in the "pxa2xx_spi_master" structure and and
+ensuring that the "pxa2xx_spi_chip.dma_burst_size" field is non-zero.  The DMA
+mode support 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 or dma_burst_size == 0 then
+        always 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.
+