linux-kernel - Re: [PATCH 02/55] media: test-drivers: Stop abusing of min_buffers

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Date:   Tue, 28 Nov 2023 10:27:52 +0100
From:   Benjamin Gaignard <benjamin.gaignard@...labora.com>
To:     Shuah Khan <skhan@...uxfoundation.org>, hverkuil@...all.nl,
        mchehab@...nel.org, tfiga@...omium.org, m.szyprowski@...sung.com,
        matt.ranostay@...sulko.com
Cc:     linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-staging@...ts.linux.dev, kernel@...labora.com,
        Kieran Bingham <kieran.bingham@...asonboard.com>
Subject: Re: [PATCH 02/55] media: test-drivers: Stop abusing of
 min_buffers_needed field


Le 27/11/2023 à 18:00, Shuah Khan a écrit :
> On 11/27/23 09:54, Benjamin Gaignard wrote:
>> 'min_buffers_needed' is suppose to be used to indicate the number
>> of buffers needed by DMA engine to start streaming.
>> Obvious test-drivers don't use DMA engine and just want to specify
>> the minimum number of buffers to allocate when calling VIDIOC_REQBUFS.
>> That 'min_reqbufs_allocation' field purpose so use it.
>> While at it rename function parameter.
>>
>
> So how ate the test-drivers currently abusing this field
> and how does this change benefit?

In videobuf2-core.h min_buffers_needed is documemented as:
"the minimum number of buffers needed before @start_streaming can be called.
Used when a DMA engine cannot be started unless at least this number of buffers have been queued into the driver."

Test-drivers don't use DMA engine so they (ab)use of this variable, like Hans said in this RFC:
https://www.spinics.net/lists/linux-media/msg244455.html

The goal of this series is to distinguish the minimum number of buffers to be allocated in VIDIOC_REQBUF
versus the minimum numbers of buffers needed before start streaming.

Regards,
Benjamin

>
> I don't have objections to this change, I want to understand
> it a bit more. I would like to see more details on why this
> change is needed.

>
> thanks,
> -- Shuah