linux-kernel - Re: [PATCH v1 5/5] gpiolib: cdev: Utilize more bitmap APIs

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Date:   Mon, 2 Oct 2023 17:25:05 +0800
From:   Kent Gibson <warthog618@...il.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
        Yury Norov <yury.norov@...il.com>, linux-gpio@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Shubhrajyoti Datta <shubhrajyoti.datta@....com>,
        Srinivas Neeli <srinivas.neeli@....com>,
        Michal Simek <michal.simek@....com>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Marek Behún <kabel@...nel.org>
Subject: Re: [PATCH v1 5/5] gpiolib: cdev: Utilize more bitmap APIs

On Mon, Oct 02, 2023 at 12:05:11PM +0300, Andy Shevchenko wrote:
> On Wed, Sep 27, 2023 at 10:23:12PM +0800, Kent Gibson wrote:
> > On Wed, Sep 27, 2023 at 04:59:34PM +0300, Andy Shevchenko wrote:
> > > On Wed, Sep 27, 2023 at 09:49:35PM +0800, Kent Gibson wrote:
> > > > On Wed, Sep 27, 2023 at 03:17:06PM +0300, Andy Shevchenko wrote:
> > > > > On Wed, Sep 27, 2023 at 09:32:11AM +0800, Kent Gibson wrote:
> 
> ...
> 
> > > > > Yet, it opens a way to scale this in case we might have v3 ABI that let's say
> > > > > allows to work with 512 GPIOs at a time. With your code it will be much harder
> > > > > to achieve and see what you wrote about maintenance (in that case).
> > > > 
> > > > v3 ABI?? libgpiod v2 is barely out the door!
> > > > Do you have any cases where 64 lines per request is limiting?
> > > 
> > > IIRC it was SO question where the OP asks exactly about breaking the 64 lines
> > > limitation in the current ABI.
> > > 
> > > > If that sort of speculation isn't premature optimisation then I don't know
> > > > what is.
> > > 
> > > No, based on the real question / discussion, just have no link at hand.
> > > But it's quite a niche, I can agree.
> > 
> > Let me know if you find a ref to that discussion - I'm curious.
> 
> Here it is (read comments as well):
> https://stackoverflow.com/questions/76307370/control-gpio-from-linux-userspace-with-linux-gpio-h
> 

That question looks to me to be confusing how many GPIOs can be
requested per request (64) and in total (effectively unlimited) - thinking
they are the same.
That could be due to their desire to use the gpiod_chip_get_all_lines()
convenience function with a chip with more than 64 lines, rather than
because they have an actual need for the lines to be managed in a single
request.

So that doesn't look like a genuine use case to me - just a "what if I
want to do X" question.  Certainly not something that would warrant a v3
ABI.

Cheers,
Kent.