linux-kernel - Re: [PATCH v5] Driver for ON Semi AR0521 camera sensor

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Date:   Thu, 14 Oct 2021 12:43:24 +0200
From:   Krzysztof Hałasa <khalasa@...p.pl>
To:     Jacopo Mondi <jacopo@...ndi.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Sakari Ailus <sakari.ailus@....fi>,
        Matteo Lisi <matteo.lisi@...icam.com>
Subject: Re: [PATCH v5] Driver for ON Semi AR0521 camera sensor

Jacopo Mondi <jacopo@...ndi.org> writes:

>> This is BTW completely orthogonal to the -EBUSY on set_fmt(). The
>> effects will be exactly the same if the e.g. geometry changes come when
>> the sensor is not streaming.
>>
>
> No, this isn't true. Your s_fmt() implementation stops then restart the
> stream. It has an undocumented side effect and will cause undefined
> behaviour.

It will cause *at*most* a corrupted frame. On a MIPI link. That's right.
Such a corrupted frame will *at*most* cause some transient IO error - it
must not cause anything serious, because corrupted frames on MIPI can
happen for multiple reasons, some of which simply cannot be avoided.
BTW I will see if it's actually the case - chances are, there is no
corruption, but I tested it years ago and haven't yet checked my notes.

In fact those set_fmt() in other drivers may - or may not - cause
corrupted frames just the same.

> If your s_fmt() has to stop and restart streaming to take effect,
> it means userspace should instead stop the stream, change
> the format where opportune in the pipeline, and then restart the
> stream.

Maybe. This is a sensor driver - not userspace.
If the userspace uses it as a part of "frame grabber", it will certainly
do exactly that (nothing else would make sense in practice).

Unfortunately this all will have to wait a bit, so thanks for your help,
expect a new patch in few weeks.
-- 
Krzysztof "Chris" Hałasa

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