linux-kernel - Re: [PATCH 0/4] gpiolib: cdev: relocate debounce_period

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Date:   Wed, 13 Dec 2023 11:03:40 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Kent Gibson <warthog618@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-gpio@...r.kernel.org,
        linus.walleij@...aro.org, andy@...nel.org
Subject: Re: [PATCH 0/4] gpiolib: cdev: relocate debounce_period_us

On Wed, Dec 13, 2023 at 12:58 AM Kent Gibson <warthog618@...il.com> wrote:
>
> On Tue, Dec 12, 2023 at 06:09:00PM +0100, Bartosz Golaszewski wrote:

[snip]

> >
> > Patches 2-4 look fine, I was about to review patch 1 in detail but I
> > thought I'd just throw this one in here before we commit to a specific
> > solution.
> >
> > For some reason I thought this would not work but I'm now considering
> > it as an alternative approach: is there anything wrong with adding
> > struct kref to struct line, allocating it separately per-line when
> > gpio_chardev_data is created, referencing it from struct linereq when
> > the line is being requested, and dropping the reference from
> > gpio_chardev_data and linereq when either is being removed? Other than
> > the increased number of allocations?
> >
>
> The collection of struct line always has to be global, right, as both
> gpio_chardev_data and linereq are ephemeral.  e.g. if one process requests
> a line and another checks the lineinfo, those will have distinct
> gpio_chardev_data.
>

Strictly speaking at least the supplemental info has to be globally
accessible. But I get your point.

> But the key issue is that the linereq and struct line lifetimes are
> strictly tied - a struct line does not live beyond the containing linereq.

I was thinking about decoupling one from the other actually.

> Leaving the struct line alive after the linereq is released is just wrong.
> The line has been released back to gpiolib so there can be no
> supplemental info left.

Indeed.

> If you want any such info to persist beyond the line release then it
> should be located in gpiolib itself, not cdev.
>

Right, we even zero debounce_period_us anyway on line release - just
as if we released struct line.

Bart