linux-kernel - Re: [PATCH v2 2/4] media: uvcvideo: Do not set an async control owned by other fh

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Message-ID: <CANiDSCvwzY3DJ+U3EyzA7TCQu2qMUL6L1eTmZYbM+_Tk6DsPaA@mail.gmail.com>
Date: Fri, 29 Nov 2024 19:47:31 +0100
From: Ricardo Ribalda <ribalda@...omium.org>
To: Hans Verkuil <hverkuil-cisco@...all.nl>
Cc: Laurent Pinchart <laurent.pinchart@...asonboard.com>, Hans de Goede <hdegoede@...hat.com>, 
	Mauro Carvalho Chehab <mchehab@...nel.org>, 
	Guennadi Liakhovetski <guennadi.liakhovetski@...el.com>, 
	Mauro Carvalho Chehab <mchehab+samsung@...nel.org>, linux-media@...r.kernel.org, 
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/4] media: uvcvideo: Do not set an async control owned
 by other fh

Before we all go on a well deserved weekend, let me recap what we
know. If I did not get something correctly, let me know.

1) Well behaved devices do not allow to set or get an incomplete async
control. They will stall instead (ref: Figure 2-21 in UVC 1.5 )
2) Both Laurent and Ricardo consider that there is a big chance that
some camera modules do not implement this properly. (ref: years of
crying over broken module firmware :) )
3) ctrl->handle is designed to point to the fh that originated the
control. So the logic can decide if the originator needs to be
notified or not. (ref: uvc_ctrl_send_event() )
4) Right now we replace the originator in ctrl->handle for unfinished
async controls.  (ref:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/media/usb/uvc/uvc_ctrl.c#n2050)

My interpretation is that:
A) We need to change 4). We shall not change the originator of
unfinished ctrl->handle.
B) Well behaved cameras do not need the patch "Do not set an async
control owned by another fh"
C) For badly behaved cameras, it is fine if we slightly break the
v4l2-compliance in corner cases, if we do not break any internal data
structure.

I will send a new version with my interpretation.

Thanks for a great discussion