linux-kernel - Re: [PATCH 07/55] media: imx8-isi: Stop abusing of min_buffers

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Date:   Tue, 28 Nov 2023 11:26:40 +0100
From:   Benjamin Gaignard <benjamin.gaignard@...labora.com>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        hverkuil@...all.nl, mchehab@...nel.org, m.szyprowski@...sung.com,
        matt.ranostay@...sulko.com, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, linux-staging@...ts.linux.dev,
        kernel@...labora.com, Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>
Subject: Re: [PATCH 07/55] media: imx8-isi: Stop abusing of min_buffers_needed
 field


Le 28/11/2023 à 10:35, Tomasz Figa a écrit :
> On Tue, Nov 28, 2023 at 6:31 PM Benjamin Gaignard
> <benjamin.gaignard@...labora.com> wrote:
>>
>> Le 27/11/2023 à 18:07, Laurent Pinchart a écrit :
>>> Hi Benjamin,
>>>
>>> Thank you for the patch.
>>>
>>> On Mon, Nov 27, 2023 at 05:54:06PM +0100, Benjamin Gaignard wrote:
>>>> 'min_buffers_needed' is suppose to be used to indicate the number
>>>> of buffers needed by DMA engine to start streaming.
>>>> imx8-isi driver doesn't use DMA engine and just want to specify
>>> What do you mean, "doesn't use DMA engine" ? The ISI surely has DMA
>>> engines :-)
>> I have done assumption on drivers given if they use or dma_* functions.
> I suspect the use of vb2_dma_sg_plane_desc() and
> vb2_dma_contig_plane_dma_addr() may be more correlated to whether
> there is a DMA involved or not. Usually V4L2 drivers don't really have
> to deal with the DMA API explicitly, because the vb2 framework handles
> most of the work.

Unfortunately isn't not true either, for example verisilicon driver use
these function but doesn't need DMA engine.
I haven't found yet a 100% criteria to decide if driver use or not DMA engine
so I plan to fix case by case given maintainers remarks.

Regards,
Benjamin

>
> Best regards,
> Tomasz
>
>> I have considers that all PCI drivers are using DMA engine and
>> I don't know the design for each drivers so I hope to get this information
>> from maintainers and fix that in v2.
>> If imx8-isi driver needs a minimum number of buffers before start streaming
>> I will do a v2 and use min_dma_buffers_needed instead.
>>
>> Regards,
>> Benjamin
>>
>>>> the minimum number of buffers to allocate when calling VIDIOC_REQBUFS.
>>>> That 'min_reqbufs_allocation' field purpose so use it.
>>>>
>>>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>
>>>> CC: Laurent Pinchart <laurent.pinchart@...asonboard.com>
>>>> CC: Mauro Carvalho Chehab <mchehab@...nel.org>
>>>> CC: Shawn Guo <shawnguo@...nel.org>
>>>> CC: Sascha Hauer <s.hauer@...gutronix.de>
>>>> CC: Pengutronix Kernel Team <kernel@...gutronix.de>
>>>> CC: Fabio Estevam <festevam@...il.com>
>>>> CC: NXP Linux Team <linux-imx@....com>
>>>> ---
>>>>    drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c b/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c
>>>> index 49bca2b01cc6..81673ff9084b 100644
>>>> --- a/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c
>>>> +++ b/drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c
>>>> @@ -1453,7 +1453,7 @@ int mxc_isi_video_register(struct mxc_isi_pipe *pipe,
>>>>       q->mem_ops = &vb2_dma_contig_memops;
>>>>       q->buf_struct_size = sizeof(struct mxc_isi_buffer);
>>>>       q->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
>>>> -    q->min_buffers_needed = 2;
>>>> +    q->min_reqbufs_allocation = 2;
>>>>       q->lock = &video->lock;
>>>>       q->dev = pipe->isi->dev;
>>>>