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

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Date:   Wed, 13 Dec 2023 21:17:47 +0800
From:   Kent Gibson <warthog618@...il.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
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 11:03:40AM +0100, Bartosz Golaszewski wrote:
> 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.
>

I was also headed down that path - making the supplemental info for each
line distinct from the struct line.  But then I realised that the lifetime
is strictly tied to the linereq, as per the struct line, and there was no
benefit in a separate object - just more overhead.

Cheers,
Kent.