How's the Major number allocated for platform device driver?

Question

I wonder how the major number is allocated for platform device driver. For example, in the driver code, I don't see any of the function calls like

alloc_chrdev_region()

or

register_chrdev_region()

Somebody, please make me understand this. Thank you.

Answer 1

Kernel creates a great deal of devices attached to various virtual buses (which may or may not represent a physical one). Only some of those devices can be meaningfully accessed directly from user space. And only a subset of those relies on "device node" interface to do so (as ample other options exist in modern kernels). If this particular interface is not used by a driver, then there's no need whatsoever to allocate device node numbers.

Inside the kernel devices are located by their affiliation to particular buses (using internal device names and bus ids). For example, mcspi driver registers as "device" on "platform bus" and as "bus master" on "spi bus". Upon seeing that bus master had registered, spi subsystem will trigger a "bus rescan" on a newly connected bus.

The spidev driver is rigged in such a way as to always "match" an imaginary device present on every spi bus, so it will get instantiated for every "bus master" registration. It will create the user space device node which can be used for direct communication with its "bus master" (spi bus controller, mcspi in this particular case).

Answer 2

Controller doesn't need to be exposed. Hence, no device nos.

On the other hand, SPI devices do require a MAJOR?MINOR no defined in spidev.c, here it is registering the device. And on the top of the same file, there's a macro for the major no defined as:

56 #define SPIDEV_MAJOR 153 /* assigned */
57 #define N_SPI_MINORS 32 /* ... up to 256 */