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Date:   Tue, 11 Apr 2023 15:51:36 +0900
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Subject: Re: [PATCH v1] media: vivid: Add webcam parameter for (un)limited
 bandwidth
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To:     Mauro Carvalho Chehab <mchehab@kernel.org>
Cc:     Hans Verkuil <hverkuil@xs4all.nl>, linux-media@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Ricardo Ribalda <ribalda@chromium.org>,
        Yunke Cao <yunkec@chromium.org>,
        Tomasz Figa <tfiga@chromium.org>
References: <20230410063356.3894767-1-mstaudt@chromium.org>
 <20230410102350.382f7d02@sal.lan>
From:   Max Staudt <mstaudt@chromium.org>
In-Reply-To: <20230410102350.382f7d02@sal.lan>
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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