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

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Message-ID: <ZRPQPL97T2DXbAHv@sol>
Date:   Wed, 27 Sep 2023 14:48:28 +0800
From:   Kent Gibson <warthog618@...il.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
        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>,
        Andy Shevchenko <andy@...nel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Marek Behún <kabel@...nel.org>
Subject: Re: [PATCH v1 5/5] gpiolib: cdev: Utilize more bitmap APIs

On Tue, Sep 26, 2023 at 05:46:07PM -0700, Yury Norov wrote:
> On Tue, Sep 26, 2023 at 08:20:07AM +0300, Andy Shevchenko wrote:
> > Currently we have a few bitmap calls that are open coded in the library
> > module. Let's convert them to use generic bitmap APIs instead.
> > 
> > Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> > +	bitmap_from_arr64(mask, &lv->mask, GPIO_V2_LINES_MAX);
> > +	bitmap_from_arr64(bits, &lv->bits, GPIO_V2_LINES_MAX);
> > +
> > +	num_set = bitmap_gather(vals, bits, mask, lr->num_lines);
> 
> It looks like GPIO_V2_LINES_MAX is always 64, and so I wonder: is
> my understanding correct that all bits in ->mask and ->bits beyond
> lr->num_lines are clear?
> 

The lv fields come from userspace and so cannot be guaranteed to be
zeroed beyond lr->num_lines.  Any set bits beyond that must be ignored,
one way or another.

> If so, you can seemingly pass the GPIO_V2_LINES_MAX instead of
> lr->num_lines, and that way it will be small_cons_nbits()-optimized.
> 

But that would be decidedly non-optimal for the most common case where
lr->num_lines == 1.

Cheers,
Kent.