@@ -2,43 +2,47 @@
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
+This a mini HOWTO on the pxa2xx_spi driver. The driver turns a PXA2xx
+synchronous serial port into an SPI master controller
(see Documentation/spi/spi-summary.rst). The driver has the following features
-- Support for any PXA2xx SSP
+- Support for any PXA2xx and compatible 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.
+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 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::
+Typically, for a legacy platform, an 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.
+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 MASTER SAMPLE
------------------
-Below is a sample configuration using the PXA255 NSSP::
+Below is a sample configuration using the PXA255 NSSP for a legacy platform::
static struct resource pxa_spi_nssp_resources[] = {
[0] = {
@@ -79,9 +83,10 @@ Below is a sample configuration using the PXA255 NSSP::
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.
+Typically, for a legacy platform, 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
@@ -101,9 +106,9 @@ device. All fields are optional.
};
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
+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::
+FIFO overruns (especially in PIO mode transfers). Good default values are::
.tx_threshold = 8,
.rx_threshold = 8,
@@ -118,7 +123,7 @@ 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
+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.
@@ -131,19 +136,19 @@ 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.
+configured to use GPIO or 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)
+asserted around the complete message. Use SSPFRM as a GPIO (through a descriptor)
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.
+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.
::
@@ -212,7 +217,9 @@ 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
+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
@@ -236,5 +243,4 @@ a per "spi_transfer" basis::
THANKS TO
---------
-
David Brownell and others for mentoring the development of this driver.
Update documentation by pointing out that it's applicable mostly for a legacy platform. While at it, add couple of points with regard to ACPI, Device Tree, and automatic DMA enablement. Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com> --- Documentation/spi/pxa2xx.rst | 58 ++++++++++++++++++++---------------- 1 file changed, 32 insertions(+), 26 deletions(-)