Using spi-bitbang driver

Question

I am writing a kernel module to read and write to SPI device (CC1200). My linux device does not have native SPI, so I am trying to bit-bang the bus.

I found that linux has built-in bitbang code (linux/spi/spi_bitbang.h), but I am confused how to set it up. It needs structs as spi_device and spi_master, each requiring struct device, which requires structs as kobject and many many more, most of them I have no idea what to do with them, and how they are needed for simple bit-banging.

I have looked online for examples, but i found literally none. Not a single use of the included bitbang code, only some references that it is "easy"

I will be very thankful for any help, maybe the bitbang lib is not even the good path. Maybe I can write my own (how to do it efficiently? I have 4 cores, but it is running lots of stuff in the background)

Thanks

Answer 1

Because of the nature of SPI where data is clocked and read by the master there is nothing wrong with the bit banging driver for the master, as the slave should not relay on a stable clock. But of course it depends on the slave device if this will work in practice or not.

If you are using the linux kernel there is no need to implement your own bit-banging driver as there already is one spi-gpio.c

My guess how to get it up and running would be by defining what GPIO pins to use in the devicetree, then the driver would be able to act as any of the other physical layer drivers.

I had a quick glance at drivers/spi/spi-gpio.c source code, and there is even a short user guide how to directly access the GPIO pins inline without using the generic GPIO layer overhead.

/*
 * Because the overhead of going through four GPIO procedure calls
 * per transferred bit can make performance a problem, this code
 * is set up so that you can use it in either of two ways:
 *
 *   - The slow generic way:  set up platform_data to hold the GPIO
 *     numbers used for MISO/MOSI/SCK, and issue procedure calls for
 *     each of them.  This driver can handle several such busses.
 *
 *   - The quicker inlined way:  only helps with platform GPIO code
 *     that inlines operations for constant GPIOs.  This can give
 *     you tight (fast!) inner loops, but each such bus needs a
 *     new driver.  You'll define a new C file, with Makefile and
 *     Kconfig support; the C code can be a total of six lines:
 *
 *    #define DRIVER_NAME  "myboard_spi2"
 *    #define  SPI_MISO_GPIO  119
 *    #define  SPI_MOSI_GPIO  120
 *    #define  SPI_SCK_GPIO   121
 *    #define  SPI_N_CHIPSEL  4
 *    #include "spi-gpio.c"
 */

PS are you sure your platform does not have spi, all the SoC I have worked with from HiSilicon have had one. I would double check this first