path: root/Documentation/spi
diff options
authorVernon Sauder <vernoninhand@gmail.com>2008-10-16 05:02:43 (GMT)
committerLinus Torvalds <torvalds@linux-foundation.org>2008-10-16 18:21:38 (GMT)
commitf1f640a9c1d97a1a131879ab1efe3766443904d7 (patch)
treea31853ea036cf9d0f42310ca60b555fe31b886e1 /Documentation/spi
parent65a00a20655f4929c4991017e230175f61c8f052 (diff)
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>
Diffstat (limited to 'Documentation/spi')
1 files changed, 23 insertions, 11 deletions
diff --git a/Documentation/spi/pxa2xx b/Documentation/spi/pxa2xx
index bbe8dee..6bb916d 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. 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.
+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.
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

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