How to write a simple Linux device driver?

Question

I need to write an SPI Linux character device driver for omap4 from scratch. I know some basics of writing device drivers. But, I don\'t know how to start writing platform s

Answer 1

I don't know if I understood your question correctly. As m-ric pointed out, there are master drivers and slave drivers.

Usually master drivers are more hardware bound, I mean, they usually manipulate IO registers or do some memory mapped IO.

For some architectures already supported by linux kernel (like omap3 and omap4) master drivers are already implemented (McSPI).

So I assume you want to USE those SPI facilities of omap4 to implement a slave device driver (your protocol, to communicate with your external device through SPI).

I've written the following example for BeagleBoard-xM (omap3). The full code is at https://github.com/rslemos/itrigue/blob/master/alsadriver/itrigue.c (worth a view, but have more initialisation code, for ALSA, GPIO, module parameters). I've tried to set apart code that deals with SPI (maybe I forgot something, but anyway you should get the idea):

#include 
#include 
#include 

/* MODULE PARAMETERS */
static uint spi_bus = 4;
static uint spi_cs = 0;
static uint spi_speed_hz = 1500000;
static uint spi_bits_per_word = 16;

/* THIS IS WHERE YOUR DEVICE IS CREATED; THROUGH THIS YOU INTERACT WITH YOUR EXTERNAL DEVICE */
static struct spi_device *spi_device;


/* SETUP SPI */

static inline __init int spi_init(void) {
    struct spi_board_info spi_device_info = {
        .modalias = "module name",
        .max_speed_hz = spi_speed_hz,
        .bus_num = spi_bus,
        .chip_select = spi_cs,
        .mode = 0,
    };

    struct spi_master *master;

    int ret;

    // get the master device, given SPI the bus number
    master = spi_busnum_to_master( spi_device_info.bus_num );
    if( !master )
        return -ENODEV;

    // create a new slave device, given the master and device info
    spi_device = spi_new_device( master, &spi_device_info );
    if( !spi_device )
        return -ENODEV;

    spi_device->bits_per_word = spi_bits_per_word;

    ret = spi_setup( spi_device );
    if( ret )
        spi_unregister_device( spi_device );

    return ret;
}

static inline void spi_exit(void) {
    spi_unregister_device( spi_device );
}

To write data to your device:

spi_write( spi_device, &write_data, sizeof write_data );

The above code is independent of implementation, that is, it could use McSPI, bit-banged GPIO, or any other implementation of an SPI master device. This interface is described in linux/spi/spi.h

To make it work in BeagleBoard-XM I had to add the following to the kernel command line:

omap_mux=mcbsp1_clkr.mcspi4_clk=0x0000,mcbsp1_dx.mcspi4_simo=0x0000,mcbsp1_dr.mcspi4_somi=0x0118,mcbsp1_fsx.mcspi4_cs0=0x0000

So that an McSPI master device is created for omap3 McSPI4 hardware facility.

Hope that helps.