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Message-ID: <7aa0e48b-6ddc-4af3-9b42-aa48c29be2e1@collabora.com>
Date:   Tue, 28 Nov 2023 10:27:52 +0100
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Subject: Re: [PATCH 02/55] media: test-drivers: Stop abusing of
 min_buffers_needed field
To:     Shuah Khan <skhan@linuxfoundation.org>, hverkuil@xs4all.nl,
        mchehab@kernel.org, tfiga@chromium.org, m.szyprowski@samsung.com,
        matt.ranostay@konsulko.com
Cc:     linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
        linux-staging@lists.linux.dev, kernel@collabora.com,
        Kieran Bingham <kieran.bingham@ideasonboard.com>
References: <20231127165454.166373-1-benjamin.gaignard@collabora.com>
 <20231127165454.166373-3-benjamin.gaignard@collabora.com>
 <dd6739ca-72b1-4eb6-9fd6-e18dc974de50@linuxfoundation.org>
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From:   Benjamin Gaignard <benjamin.gaignard@collabora.com>
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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