linux-kernel - Re: [PATCH] gpiolib: use __counted

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Message-ID: <CAMRc=MesmR+=gMsk2y0Y7jiwtFpcHCofYsi3mxWFrdY9=1Y46A@mail.gmail.com>
Date: Wed, 20 Dec 2023 13:35:37 +0100
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Linus Walleij <linus.walleij@...aro.org>, Kent Gibson <warthog618@...il.com>, 
	linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH] gpiolib: use __counted_by() for GPIO descriptors

On Wed, Dec 20, 2023 at 9:16 AM Bartosz Golaszewski <brgl@...ev.pl> wrote:
>
> On Tue, Dec 19, 2023 at 4:12 PM Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com> wrote:
> >
> > On Tue, Dec 19, 2023 at 01:57:06PM +0100, Bartosz Golaszewski wrote:
> > > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > >
> > > Pull the array of GPIO descriptors into struct gpio_device as a flexible
> > > array and use __counted_by() to control its size.
> >
> > How big is the struct gpio_device? Unifying like this might provoke subtle
> > errors on very fragmented memory, where k*alloc() might not find enough free
> > space. Note, k*alloc() guarantees finding only memory for a single page.
> > With PAGE_SIZE = 4k, this might be an issue.
> >
> > I would suggest, if nothing prevents us from switching, to use kvmalloc().
> >
>
> That's a good point but there's another thing. We need to call
> gpiochip_get_ngpios() before the allocation. I need to revisit this
> one.
>

It's trickier than I thought. We need the struct device to exist (have
its software node assigned) before we check the property but we cannot
allocate gdev (embedding struct device) + descs before we have read
it. Eh, maybe it's not worth it and let's keep the two allocations
separate.

Bart

> Bart
>
> > --
> > With Best Regards,
> > Andy Shevchenko
> >
> >