pxa2xx_spi: fix chip_info defaults and documentation.

Make the chip info structure data optional by providing reasonable
defaults.  Improve corresponding documentation, and highlight the drawback
of not providing explicit chipselect control.

DMA can determine appropriate dma_burst_size and thresholds automatically
so use DMA even if dma_burst_size is not specified.

Signed-off-by: Vernon Sauder <VernonInHand@gmail.com>
Reviewed-by: Ned Forrester <nforrester@whoi.edu>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 23 insertions, 11 deletions

diff --git a/Documentation/spi/pxa2xx b/Documentation/spi/pxa2xx
index bbe8dee681a5..6bb916d57c95 100644
--- a/Documentation/spi/pxa2xx
+++ b/Documentation/spi/pxa2xx
@@ -96,7 +96,7 @@ Each slave device attached to the PXA must provide slave specific configuration
 information via the structure "pxa2xx_spi_chip" found in
 "arch/arm/mach-pxa/include/mach/pxa2xx_spi.h".  The pxa2xx_spi master controller driver
 will uses the configuration whenever the driver communicates with the slave
-device.
+device. All fields are optional.
 
 struct pxa2xx_spi_chip {
         u8 tx_threshold;
@@ -112,14 +112,17 @@ 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,
+        .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.
+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
@@ -137,7 +140,13 @@ 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
+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 accomodate 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
@@ -206,18 +215,21 @@ static void __init streetracer_init(void)
 
 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
-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 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 or dma_burst_size == 0 then
+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