linux-kernel - Re: regression: gpiolib: switch the line state notifier to atomic unexpected impact on performance

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Message-ID: <20250312013256.GB27058@rigel>
Date: Wed, 12 Mar 2025 09:32:56 +0800
From: Kent Gibson <warthog618@...il.com>
To: David Jander <david@...tonic.nl>
Cc: Bartosz Golaszewski <brgl@...ev.pl>,
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
	Linus Walleij <linus.walleij@...aro.org>,
	linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: regression: gpiolib: switch the line state notifier to atomic
 unexpected impact on performance

On Tue, Mar 11, 2025 at 12:03:46PM +0100, David Jander wrote:
>
>
> Indeed, it does. My application is written in python and uses the python gpiod
> module. Even in such an environment the impact is killing.
>

Interesting - the only reason I could think of for an application
requesting/releasing GPIOs at a high rate was it if was built on top of
the libgpiod tools and so was unable to hold the request fd.
Generally an application should request the lines it requires once and hold
them for the duration.  Similarly functions such as find_line() should be
performed once per line.
>From a performance perspective, NOT having to re-request a line is
considerably faster than requesting it - even with Bart's fix.

Is there something unusual about your app that requires the lines be
released?

Cheers,
Kent.