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tidbits/oob-spi: add a few more documentation hints 

Signed-off-by: Philippe Gerum <rpm@xenomai.org>
This commit is contained in:
Philippe Gerum 2021-05-13 19:00:13 +02:00
parent edb5af7fb7
commit d12db5d268
1 changed files with 37 additions and 6 deletions
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43
tidbits/oob-spi.c
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@ -6,14 +6,20 @@
* SPIDEV interface. Typical use case: closed-loop control systems
* having stringent requirements, i.e. high frequency and/or low
* jitter. The magic starts with the ioctl(SPI_IOC_ENABLE_OOB_MODE)
* request, check out the code.
* request, check out the code below.
*
* Using a Raspberry PI2/3/4, you can easily demo this code with a
* simple loopback test, by shorting PIN #19 (SPI_MOSI) and #21
* (SPI_MISO) on the GPIO 40 pin header. You will need
* CONFIG_DMA_BCM2835_OOB, CONFIG_SPI_BCM2835_OOB and
* CONFIG_SPI_SPIDEV_OOB to be turned on in your PI kernel
* configuration.
* (SPI_MISO) on the GPIO 40 pin header, using the SPI0 controller
* (bcm2835). You need the following features to be enabled in your PI
* kernel configuration:
*
* - CONFIG_SPI=[y|m]
* - CONFIG_SPI_OOB=y
* - CONFIG_SPI_SPIDEV=[y|m]
* - CONFIG_SPI_SPIDEV_OOB=y
* - CONFIG_DMA_BCM2835=[y|m]
* - CONFIG_DMA_BCM2835_OOB=y
*
* Usage:
* ~# oob-spi [/dev/spidevX.Y]
@ -24,6 +30,31 @@
*
* ~# dd if=/dev/zero of=/dev/null bs=128M &
* ~# while :; do hackbench; done&
*
* TROUBLESHOOTING:
*
* - if you get -ENODEV on return to ioctl(SPI_IOC_ENABLE_OOB_MODE),
* then you should suspect the DMA settings for the SPI
* controller. DMA support is required for out-of-band transfers.
*
* - if you have no /dev/spidev* devices listed although the proper
* SPI driver is enabled in the kernel configuration, then the device
* tree for the SoC may be missing a SPI(dev) slave device declaration
* in the proper SPI bus stanza. You may want to set the compatible
* string to "spidev,loopback", which is detected by the EVL-enabled
* spidev driver.
*
* spi0 {
* ...
* spidev_0: spidev@0{
* compatible = "spidev,loopback";
* reg = <0>;
* #address-cells = <1>;
* #size-cells = <0>;
* spi-max-frequency = <12500000>;
* status = "okay";
* };
* };
*/
#include <sys/ioctl.h>
@ -114,7 +145,7 @@ int main(int argc, char *argv[])
/*
* We may map the I/O area now, it is composed of two adjacent
* buffers of @len bytes (plus alignment). CUATION: the
* buffers of @len bytes (plus alignment). CAUTION: the
* mapping is always on coherent DMA memory (i.e. non-cached).
*/
iobuf = mmap(NULL, oob_setup.iobuf_len, PROT_READ|PROT_WRITE,