SPI DTS skeleton example

[bruce bushby]

Hi

I've been wanting to write an SPI skeleton driver to learn the basics. I'm hoping a driver guru could offer some guidance.

The idea is to write a skeleton driver that simply prints the chip id

Kernel: stock standard mainline 3.16.0-rc7 kernel on Olimex A20-SOM (EVK)
Device: MPU9250 Break out board

My thinking so far:

1. DTS
Add an SPI slave device to my DTS file.

I will connect the Break out board to the second SPI bus .... so need to add the slave device to SPI1

Existing SPI1 device (from dts file)

        spi1: spi@01c06000 {
            pinctrl-names = "default";
            pinctrl-0 = <&spi1_pins_a>;
            status = "okay";
        };


Adding SPI1 slave device:

        spi1: spi@01c06000 {
            pinctrl-names = "default";
            pinctrl-0 = <&spi1_pins_a>;
            status = "okay";
 
                        mpu9250@0 {
                                 compatible = "mpu9250";
                                 reg = <0>;
                                 spi-max-frequency = <1000000>;
                                 ???? interrupt pin .... gpio ???????
                         };

        };
2. The driver will create an entry in "sysfs" and a "corresponding" entry in /dev. Catting the device " /dev/mpu9250 " will return the device id.

Then the driver will be a standard Linux kernel module with support for SPI
....
#include <linux/module.h>
#include <linux/spi/spi.h>
....

struct spi_driver
spi_register_driver  (struct device dev;   dictates the name of the device)
spi_alloc_device
spi_add_device



Am I on the right track? Any similar simple examples floating about?


Thanks
Bruce

[jons...@gmail.com]

I attached the Allwinner SPI driver from the 3.4 kernel. That would be
a good place to start when modifying it for mainline.

Look at other spi drivers in the mainline kernel. drivers/spi/...
They will show you how to fix probe up to work with device trees.

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[Maxime Ripard]

This one is okay.

> 2. The driver will create an entry in "sysfs" and a "corresponding" entry
> in /dev. Catting the device " /dev/mpu9250 " will return the device id.
>
> Then the driver will be a standard Linux kernel module with support for SPI
> ....
> #include <linux/module.h>
> #include <linux/spi/spi.h>
> ....
>
> struct spi_driver
> spi_register_driver (struct device dev; dictates the name of the device)
> spi_alloc_device
> spi_add_device

Not quite.

You should declare a struct spi_driver, register it using
spi_register_module, and in that structure, put the compatibles you
support. You'll also have to implement the probe and remove functions,
give in that spi_driver structure pointers to these functions, and
Linux will call probe whenever a device shows up, and call remove
whenever that device disappears.

Note that all this is covered in LDD3 and
http://free-electrons.com/doc/training/linux-kernel/slides.pdf, so I'd
suggest you go read these two, like I suggested you already (or should
have)

As for how to find simple SPI drivers, I don't really have an example
in mind, but you can search for spi_register_module, you should have a
huge number of hits.

Maxime

--
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

[bruce bushby]

Thanks guys, I think I'm making some progress now.....if only I had more time :)

I found this little guy:

https://github.com/torvalds/linux/blob/master/drivers/hwmon/lm70.c

Only 181 lines so not too complicated, will have a go at adapting it to read the device id from my breakout board.

Also need to pop over to Emilio's repo as he was working on releasing dma enabled spi for sunxi.