linux-kernel - Re: [PATCH v2 03/36] media: microchip: Remove useless setting of min_buffers

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Date:   Mon, 4 Dec 2023 16:25:39 +0200
From:   Eugen Hristev <eugen.hristev@...labora.com>
To:     Hans Verkuil <hverkuil@...all.nl>,
        Benjamin Gaignard <benjamin.gaignard@...labora.com>,
        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
Subject: Re: [PATCH v2 03/36] media: microchip: Remove useless setting of
 min_buffers_needed

On 12/4/23 16:03, Hans Verkuil wrote:
> On 04/12/2023 14:22, Benjamin Gaignard wrote:
>> This driver uses min_buffers_needed which vb2 uses to ensure
>> start_streaming is called when at least 'min_buffers_needed'
>> buffers are queued. However, this driver doesn't need this,
>> it can stream fine without any buffers queued.
>> Just drop this unnecessary restriction.
>>
>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>
>> CC: Eugen Hristev <eugen.hristev@...labora.com>
>> ---
>>  drivers/media/platform/microchip/microchip-isc-base.c | 1 -
>>  1 file changed, 1 deletion(-)
>>
>> diff --git a/drivers/media/platform/microchip/microchip-isc-base.c b/drivers/media/platform/microchip/microchip-isc-base.c
>> index 3fba0e2844b6..63c39981f47a 100644
>> --- a/drivers/media/platform/microchip/microchip-isc-base.c
>> +++ b/drivers/media/platform/microchip/microchip-isc-base.c
>> @@ -1821,7 +1821,6 @@ static int isc_async_complete(struct v4l2_async_notifier *notifier)
>>  	q->mem_ops		= &vb2_dma_contig_memops;
>>  	q->timestamp_flags	= V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
>>  	q->lock			= &isc->lock;
>> -	q->min_buffers_needed	= 1;
> 
> I don't think this can be dropped. Looking at the isc_start_streaming() function
> it expects to have at least one buffer queued.

Stream goes frame by frame with DMA directly into the buffers.
Will this still work without any buffers queued ?

> 
> Regards,
> 
> 	Hans
> 
>>  	q->dev			= isc->dev;
>>  
>>  	ret = vb2_queue_init(q);
>