Re: [PATCH v2 10/11] gpiolib: add new ioctl() for monitoring changes in line info

From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Bartosz Golaszewski <brgl@bgdev.pl>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>,
	Kent Gibson <warthog618@gmail.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	"open list:GPIO SUBSYSTEM" <linux-gpio@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 10/11] gpiolib: add new ioctl() for monitoring changes in line info
Date: Tue, 10 Dec 2019 19:00:07 +0200	[thread overview]
Message-ID: <CAHp75VfeEB5RudwMaoiMTMMY3zW-kz-h=rJ3Cu5_tyRL6ZuF1w@mail.gmail.com> (raw)
In-Reply-To: <CAMRc=MfxteQDmeZn_Et0uFs2cPvLGpJ5BTeBOn37o=2Oo_eU=Q@mail.gmail.com>

On Fri, Dec 6, 2019 at 11:20 PM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> czw., 5 gru 2019 o 18:02 Andy Shevchenko <andy.shevchenko@gmail.com> napisał(a):
> > On Thu, Dec 5, 2019 at 3:47 PM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> > > czw., 5 gru 2019 o 11:27 Andy Shevchenko <andy.shevchenko@gmail.com> napisał(a):
> > > > On Thu, Dec 5, 2019 at 11:42 AM Bartosz Golaszewski
> > > > <bgolaszewski@baylibre.com> wrote:
> > > > > śr., 4 gru 2019 o 23:34 Andy Shevchenko <andy.shevchenko@gmail.com> napisał(a):
> > > > > > On Wed, Dec 4, 2019 at 6:03 PM Bartosz Golaszewski <brgl@bgdev.pl> wrote:
> >
> > > > So, the test cases, I can imagine of, should include (k - kernel, u - user):
> > > > - 64k-64u: LE and BE
> > > > - 64k-32u: LE and BE
> > > > - 32k-32u: LE and BE
> > >
> > > I usually use qemu VMs built with yocto for testing but I don't see
> > > any way of creating a 32-bit user-space with 64-bit kernel. Any ideas
> > > on how to prepare a testing environment?
> >
> > In my case it's very easy. I do
> > - compile kernel as 64-bit separately;
> > - compile initramfs of Buildroot distro with external kernel build provided.
> >
>
> Any specific config options are needed on x86-64 kernel to use 32-bit
> user-space? I'm not well versed in x86 architectures, that's why I'm
> asking. I built a 32-bit userspace qemu image with yocto and then
> manually built a 64-bit kernel. I tried running it but I'm getting a
> kernel panic when the rootfs is being mounted.

Just published set of scripts [1] we are using internally for our development.
Find README.coreteam in the source root and read how to use that.

> On a different note: why would endianness be an issue here? 32-bit
> variables with 64-bit alignment should still be in the same place in
> memory, right?

With explicit padding, yes.

> Any reason not to use __packed for this structure and not deal with
> this whole compat mess?

Have been suggested that explicit padding is better approach.
(See my answer to Kent)

> I also noticed that my change will only allow user-space to read one
> event at a time which seems to be a regression with regard to the
> current implementation. I probably need to address this too.

Yes, but we have to have ABI v2 in place.

[1]: https://github.com/andy-shev/buildroot/tree/intel

-- 
With Best Regards,
Andy Shevchenko