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

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Date:   Tue, 28 Nov 2023 12:31:17 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Benjamin Gaignard <benjamin.gaignard@...labora.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 06:35:51PM +0900, Tomasz Figa wrote:
> On Tue, Nov 28, 2023 at 6:31 PM Benjamin Gaignard 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.

And this is anyway not related to DMA at all, but to the logic each
driver implements when it deals with buffers. There's a lower chance a
USB driver driver will have a hard requirement for more than one buffer
compared to an AMBA/platform/PCI device driver, but at the end of the
day, each driver needs to be analyzed individually to check what they
require. Benjamin, I think you'll have some more homework to do :-)

> > 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.
> >
> > >> 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;
> > >>

-- 
Regards,

Laurent Pinchart