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Message-ID: <3ab3f9ea25d222c0b27e1b76f80c1abe76d27def.camel@ndufresne.ca>
Subject: Re: [PATCH 07/55] media: imx8-isi: Stop abusing of
 min_buffers_needed field
From:   Nicolas Dufresne <nicolas@ndufresne.ca>
To:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Tomasz Figa <tfiga@chromium.org>
Cc:     Benjamin Gaignard <benjamin.gaignard@collabora.com>,
        hverkuil@xs4all.nl, mchehab@kernel.org, m.szyprowski@samsung.com,
        matt.ranostay@konsulko.com, linux-kernel@vger.kernel.org,
        linux-media@vger.kernel.org, linux-staging@lists.linux.dev,
        kernel@collabora.com, Shawn Guo <shawnguo@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        Fabio Estevam <festevam@gmail.com>,
        NXP Linux Team <linux-imx@nxp.com>
Date:   Thu, 07 Dec 2023 13:33:38 -0500
In-Reply-To: <20231128103117.GF31314@pendragon.ideasonboard.com>
References: <20231127165454.166373-1-benjamin.gaignard@collabora.com>
         <20231127165454.166373-8-benjamin.gaignard@collabora.com>
         <20231127170700.GC31314@pendragon.ideasonboard.com>
         <6fa1ec09-3e30-475e-9718-29d23586753e@collabora.com>
         <CAAFQd5DCVTLpPoKSp_OA6fe_Hqt-oV7=AsCZWSmkJORvLSgUiw@mail.gmail.com>
         <20231128103117.GF31314@pendragon.ideasonboard.com>
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Le mardi 28 novembre 2023 =C3=A0 12:31 +0200, Laurent Pinchart a =C3=A9crit=
=C2=A0:
> On Tue, Nov 28, 2023 at 06:35:51PM +0900, Tomasz Figa wrote:
> > On Tue, Nov 28, 2023 at 6:31=E2=80=AFPM Benjamin Gaignard wrote:
> > > Le 27/11/2023 =C3=A0 18:07, Laurent Pinchart a =C3=A9crit :
> > > > Hi Benjamin,
> > > >=20
> > > > Thank you for the patch.
> > > >=20
> > > > 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 :-)
> > >=20
> > > I have done assumption on drivers given if they use or dma_* function=
s.
> >=20
> > 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.
>=20
> 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 :-)

Personally, I disagree that we should blindly patch drivers and actually ch=
ange
the framework behaviour. A patch that simply take what we have, and make it=
 so a
human reader now understand what is going on should be acceptable. Maintain=
ers
or developer that have access to the hardware should be making this judgmen=
t now
that the current behaviour is visible, modify and test it.

Asking to eye review drivers and change their behaviour without any test be=
ing
conducted will certainly cause regressions. I don't believe this is the rig=
ht
approach. Refactoring the code, making an effort to not change the behaviou=
r
during refactoring does make more sense to me.

regards,
Nicolas

>=20
> > > 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 inform=
ation
> > > from maintainers and fix that in v2.
> > > If imx8-isi driver needs a minimum number of buffers before start str=
eaming
> > > I will do a v2 and use min_dma_buffers_needed instead.
> > >=20
> > > > > the minimum number of buffers to allocate when calling VIDIOC_REQ=
BUFS.
> > > > > That 'min_reqbufs_allocation' field purpose so use it.
> > > > >=20
> > > > > Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com=
>
> > > > > CC: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > > > > CC: Mauro Carvalho Chehab <mchehab@kernel.org>
> > > > > CC: Shawn Guo <shawnguo@kernel.org>
> > > > > CC: Sascha Hauer <s.hauer@pengutronix.de>
> > > > > CC: Pengutronix Kernel Team <kernel@pengutronix.de>
> > > > > CC: Fabio Estevam <festevam@gmail.com>
> > > > > CC: NXP Linux Team <linux-imx@nxp.com>
> > > > > ---
> > > > >   drivers/media/platform/nxp/imx8-isi/imx8-isi-video.c | 2 +-
> > > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > > >=20
> > > > > 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_p=
ipe *pipe,
> > > > >      q->mem_ops =3D &vb2_dma_contig_memops;
> > > > >      q->buf_struct_size =3D sizeof(struct mxc_isi_buffer);
> > > > >      q->timestamp_flags =3D V4L2_BUF_FLAG_TIMESTAMP_MONOTONIC;
> > > > > -    q->min_buffers_needed =3D 2;
> > > > > +    q->min_reqbufs_allocation =3D 2;
> > > > >      q->lock =3D &video->lock;
> > > > >      q->dev =3D pipe->isi->dev;
> > > > >=20
>=20