Re: [PATCH 5/5] gpiolib: notify user-space about in-kernel line state changes

[Kent Gibson]

On Fri, Oct 04, 2024 at 04:43:26PM +0200, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
>
> We currently only notify user-space about line config changes that are
> made from user-space. Any kernel config changes are not signalled.
>
> Let's improve the situation by emitting the events closer to the source.
> To that end let's call the relevant notifier chain from the functions
> setting direction, gpiod_set_config(), gpiod_set_consumer_name() and
> gpiod_toggle_active_low(). This covers all the options that we can
> inform the user-space about.
>
> There is a problem with gpiod_direction_output/input(), namely the fact
> that they can be called both from sleeping as well as atomic context. We
> cannot call the blocking notifier from atomic and we cannot switch to
> atomic notifier because the pinctrl functions we call higher up the stack
> take a mutex. Let's instead use a workqueue and schedule a task to emit
> the event from process context on the unbound system queue for minimal
> latencies.
>

So now there is a race between the state of the desc changing and the
notified reading it?

Cheers,
Kent.

> In tests, I typically get around 5 microseconds between scheduling the
> work and it being executed. That could of course differ during heavy
> system load but in general it should be enough to not miss direction
> change events which typically are not in hot paths.
>
> Let's also add non-notifying wrappers around the direction setters in
> order to not emit superfluous reconfigure events when requesting the
> lines.
>
> We can also now make gpiod_line_state_notify() static.
>
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>