what is the use of "of_match_ptr()" in device drivers

Question

During my work in device drivers, I came to know that of_match_table() is to point the tuple of compatibility strings.

However, I couldn't find the importance of of_match_ptr().

Answer 1

of_match_ptr() is a preprocessor macro defined in include/linux/of.h:

#ifdef CONFIG_OF
#define of_match_ptr(_ptr)    (_ptr)
#else /* CONFIG_OF */
#define of_match_ptr(_ptr)    NULL
#endif /* CONFIG_OF */

When the kernel is compiled with Device Tree and Open Firmware support, i.e. CONFIG_OF=y it will result in the passed pointer. Otherwise it will yield NULL.

Using the macro instead of directly passing the pointer, is to guard the of_match_table member. There are bus drivers which check of_match_table for NULL to not dereference an invalid pointer.

However, many drivers are compiled in-tree with a compile switch defined in their corresponding Kconfig that already depends on CONFIG_OF. For example, the compile switch of a driver for analog device ADV748X video decoder has this dependency:

config VIDEO_ADV748X
    tristate "Analog Devices ADV748x decoder"
    depends on VIDEO_V4L2 && I2C
    depends on OF
    select MEDIA_CONTROLLER
    select VIDEO_V4L2_SUBDEV_API
    select REGMAP_I2C
    select V4L2_FWNODE
    help
      V4L2 subdevice driver for the Analog Devices
      ADV7481 and ADV7482 HDMI/Analog video decoders.

      To compile this driver as a module, choose M here: the
      module will be called adv748x.

and omits the macro in drivers/media/i2c/adv748x/adv748x-core.c:

static struct i2c_driver adv748x_driver = {
    .driver = {
        .name = "adv748x",
        .of_match_table = adv748x_of_table,
        .pm = &adv748x_pm_ops,
    },
    .probe_new = adv748x_probe,
    .remove = adv748x_remove,
};