CODEWITHSUNDEEP
clinuxdriverlinux-device-driver
James Kent
James Kent

Reputation: 5933

linux inter driver communication

on a raspberry pi I want to be able to use a rotary encoder as volume control. the best way I can see to do that would be using the rotary_encoder module to read the encoder, set it up as a relative axis (so would get +1 or -1 events) and then make a driver to interpret that to create volume up or volume down keypresses.

however I'm struggling to get my head around interrupts and drivers, so my driver would have to register itself as able to provide EV_KEY events, which I think I've got handled, but I'm unsure how I can catch and act on events from the other driver?

I figured I would have to poll the /dev/input/event object created by the other driver, but I can't seem to find a guide on how to do this?

is this the best way to go? is there a way i can actually catch interrupts created by the other driver feeding events into the input system?

Upvotes: 1

Views: 2566

Answers (1)

rodrigo
rodrigo

Reputation: 98328

It seems you want to receive input events from an unrelated module. But you cannot read /dev/input/event* from kernel space, so you have to options:

  1. Write a user mode daemon that reads the /dev/input/event* and forwards to your driver, maybe with a char device or a sysfs parameter.
  2. Hook on the input driver events from your driver.

Option 1 should be straightforward. I'll elaborate on option 2.

To hook an input device from kernel you use function input_register_handler() from your module init function (and input_unregister_handler() from your exit function, of course).

This function takes a struct input_handler as argument, it has a lot of members, but you probably only need to fill name, id_table, connect, disconnect and event.

Then, in your connect callback you call input_register_handle() (note the handle vs handler names) and input_open_device() and you will get input events in your event callback.

Of course, do not forget to undo that work in your disconnect callback.

There are several instances of this API usage in the kernel, but by far the easier one to read is the evbug driver: it just dumps all input events into the kernel log.

Upvotes: 2

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