linux-kernel - Re: [PATCH v1] media: vivid: Add webcam parameter for (un)limited bandwidth

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Message-ID: <6aafad18-13a2-ef45-48a1-1f094554af31@chromium.org>
Date:   Tue, 11 Apr 2023 15:51:36 +0900
From:   Max Staudt <mstaudt@...omium.org>
To:     Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     Hans Verkuil <hverkuil@...all.nl>, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Ricardo Ribalda <ribalda@...omium.org>,
        Yunke Cao <yunkec@...omium.org>,
        Tomasz Figa <tfiga@...omium.org>
Subject: Re: [PATCH v1] media: vivid: Add webcam parameter for (un)limited
 bandwidth

Thank you Mauro for having a first look!

Questions below.

On 4/10/23 18:23, Mauro Carvalho Chehab wrote:
> IMO, instead of a parameter that just enables/disables the bandwidth
> limit, the best would be to have a parameter specifying the bandwidth
> (with 0 meaning unlimited).
> 
> If not used, vivid would initialize it to dev->webcam_bandwidth_limit,
> so a read operation will show the current limit.
Up until now, the bandwidth limit is a rather arbitrary reduction of two 
interval sizes per frame size.

How would you prefer to define a limited bandwidth in this parameter? 
How would it affect the simulated camera, do you have a suggestion for a 
formula from bandwidth to frame/interval sizes offered?

>> +/* Default: limited webcam bandwidth */
>> +static bool webcam_bandwidth_limit[VIVID_MAX_DEVS] = { [0 ... (VIVID_MAX_DEVS - 1)] = true };
>> +module_param_array(webcam_bandwidth_limit, bool, NULL, 0444);
> 
> I would also use 0666, to allow changing this on runtime.

I guess that's possible, though it would add complexity.

Currently we can ask for two instances, each with a different setting:

   n_devs=2 webcam_bandwidth_limit=1,0

This creates /dev/video0 which is limited, and /dev/video4 which is 
unlimited.

Maybe this already sufficiently covers the case you are looking for, and 
we can keep the complexity low? A real webcam won't suddenly offer new 
frame rates either...

Max