Reusing GPIOs with the net/phylink/sfp

[Jan Kundrát <jan.kundrat@xxxxxxxxx>]

Hi,
the kernel recently gained support for SFP cages. These cages use a few 
GPIOs for signaling some out-of-band information ("input signal lost", "tx 
laser failure", etc). There is also the GPIO LED trigger which can be used 
to drive a LED in response to an input GPIO's state changes. It seems that 
I cannot use both together.

I've verified that it is possible to use a GPIO LED trigger together with a 
`gpio-event-mon` userspace application, i.e., the GPIO's input is reflected 
in a LED, and the userspace can access the GPIO as an input at the same 
time.

Is it possible to use one physical GPIO pin from both the SFP driver *and* 
the GPIO LED trigger or userspace? I just wasn't able to grep/search for 
anything related to shared GPIO access.

I dug into the source a bit, and it seems that *just* calling 
request_threaded_irq(gpio_to_irq(gpio), ...) is OK (that's what the GPIO 
LED trigger is doing. On the other hand, the SFP code first requests a GPIO 
via devm_gpiod_get_optional(), then it tries to call gpiod_to_irq() and 
reverts to polling if that fails, and finally it calls 
devm_request_threaded_irq(...). Within the IRQ handler then (or in a 
delayed work thing when polling), the code calls 
gpiod_get_value_cansleep(). This means that I cannot "just unregister" the 
GPIO after the IRQ was set up :].

My end goal is to have both a LED for SFP state indication, *and* userspace 
access for reacting to that. I can already drive the LED for link up/down 
via the "netdev" trigger, but that's not all (and I am not thrilled by the 
idea of having to parse netlink messages when I already have a GPIO 
signal).

Is there a way of changing the SFP driver to support shared access to 
GPIOs? I have not found a "SHARED" flag for requesting GPIOs.

With kind regards,
Jan