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

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Date:   Tue, 28 Nov 2023 18:35:51 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Benjamin Gaignard <benjamin.gaignard@...labora.com>
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

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.

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;
> >>