linux-kernel - Re: [RESEND PATCH v6 6/7] gpiolib: add new ioctl() for monitoring changes in line info

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Date:   Wed, 12 Feb 2020 11:47:31 +0100
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Kent Gibson <warthog618@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>
Subject: Re: [RESEND PATCH v6 6/7] gpiolib: add new ioctl() for monitoring
 changes in line info

On Tue, Feb 11, 2020 at 10:19 AM Bartosz Golaszewski <brgl@...ev.pl> wrote:

> From: Bartosz Golaszewski <bgolaszewski@...libre.com>
>
> Currently there is no way for user-space to be informed about changes
> in status of GPIO lines e.g. when someone else requests the line or its
> config changes. We can only periodically re-read the line-info. This
> is fine for simple one-off user-space tools, but any daemon that provides
> a centralized access to GPIO chips would benefit hugely from an event
> driven line info synchronization.
>
> This patch adds a new ioctl() that allows user-space processes to reuse
> the file descriptor associated with the character device for watching
> any changes in line properties. Every such event contains the updated
> line information.
>
> Currently the events are generated on three types of status changes: when
> a line is requested, when it's released and when its config is changed.
> The first two are self-explanatory. For the third one: this will only
> happen when another user-space process calls the new SET_CONFIG ioctl()
> as any changes that can happen from within the kernel (i.e.
> set_transitory() or set_debounce()) are of no interest to user-space.
>
> Signed-off-by: Bartosz Golaszewski <bgolaszewski@...libre.com>

Looks good to me. This got really slim and clean after
the reviews, and I am of course also impressed by the kfifo
improvement this brings.

Reviewed-by: Linus Walleij <linus.walleij@...aro.org>

A question:

Bartosz, since you know about possible impacts on userspace,
since this code use the preferred ktime_get_ns() rather than
ktime_get_ns_real(), what happens if we just patch the other
event timestamp to use ktime_get_ns() instead, so we use the
same everywhere?

If it's fine I'd like to just toss in a patch for that as well.

Yours,
Linus Walleij