Difference between spi_driver.id_table and spi_driver.driver.of_match_table

I’m currently trying to understand how linux drivers work. As far as I know, A driver’s probe/init function is called when the kernel parses the corresponding .compatible string in the device tree. However, in the arizona-spi driver it looks like there are multiple compatible strings referenced in different members:

static const struct spi_device_id arizona_spi_ids[] = {
{ "wm5102", WM5102 },
{ "wm5110", WM5110 },
{ },
};
MODULE_DEVICE_TABLE(spi, arizona_spi_ids);

static struct spi_driver arizona_spi_driver = {
.driver = {
    .name   = "arizona",
    .owner  = THIS_MODULE,
    .pm = &arizona_pm_ops,

    // Contains e.g. "wlf,wm5102"
    .of_match_table = of_match_ptr(arizona_of_match),

},
.probe      = arizona_spi_probe,
.remove     = arizona_spi_remove,
.id_table   = arizona_spi_ids,                  // Contains "wm5102" and "wm5110"
};

This is an excerpt from here.

So what is the difference between arizona_spi_driver.id_table and arizona_spi_driver.driver.of_match_table?

Answer

There are several mechanism for driver matching. The id_table is intended to be used for finding a match from stripped device-tree entries (without vendor part), while of_match_table is used to find a match from full device-tree entries (the ones with vendor part).

If you check, arizona_of_match is defined as this:

const struct of_device_id arizona_of_match[] = {
    { .compatible = "wlf,wm5102", .data = (void *)WM5102 },
    { .compatible = "wlf,wm5110", .data = (void *)WM5110 },
    { .compatible = "wlf,wm8280", .data = (void *)WM8280 },
    { .compatible = "wlf,wm8997", .data = (void *)WM8997 },
    {},
};

wlf is the vendor part for this case, while arizona_spi_ids doesn’t contain the vendor part.

Hence, if you have something like this in your device tree:

compatible = “myvendor,wm5102”

Your device will match against id_table but not against of_match_table since the vendor is different.

The kernel will do matching against of_match_table first before check id_table (see spi_get_device_id in here). The device matching priority is: of_match_table > acpi_driver > id_table.