linux-kernel - Re: [PATCH] media: uvcvideo: Filter hw errors while enumerating controls

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Message-ID: <20241218232838.GG5518@pendragon.ideasonboard.com>
Date: Thu, 19 Dec 2024 01:28:38 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Ricardo Ribalda <ribalda@...omium.org>
Cc: Hans Verkuil <hverkuil@...all.nl>, Hans de Goede <hdegoede@...hat.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: uvcvideo: Filter hw errors while enumerating
 controls

On Wed, Dec 18, 2024 at 03:38:34PM +0100, Ricardo Ribalda wrote:
> On Fri, 13 Dec 2024 at 12:21, Ricardo Ribalda <ribalda@...omium.org> wrote:
> >
> > To implement VIDIOC_QUERYCTRL, we need to read from the hardware all the
> > values that were not cached previously. If that read fails, we used to
> > report back the error to the user.
> >
> > Unfortunately this does not play nice with userspace. When they are
> > enumerating the contols, the only expect an error when there are no
> > "next" control.
> >
> > This is probably a corner case, and could be handled in userspace, but
> > both v4l2-ctl and yavta fail to enumerate all the controls if we return
> > then -EIO during VIDIOC_QUERYCTRL. I suspect that there are tons of
> > userspace apps handling this wrongly as well.
> 
> Actually it CANNOT be handled in userspace.
> 
> If we return anything different than 0, the structure is not copied to
> userspace:

That could be fixed, we do copy data back to userspace in case of
failure for some ioctls. I don't think that would be needed though, I
believe we can either mark controls as broken in the uvcvideo driver
through quirks, or in a dynamic fashion.

> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/media/v4l2-core/v4l2-ioctl.c#n2929
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/media/v4l2-core/v4l2-ioctl.c#n3490

-- 
Regards,

Laurent Pinchart