linux-kernel - Re: [PATCHv3 1/1 RESEND] media: imx: Round line size to 4 bytes

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Date:   Thu, 24 Nov 2022 13:35:33 +0100
From:   Dorota Czaplejewicz <dorota.czaplejewicz@...i.sm>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Steve Longerbeam <slongerbeam@...il.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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>,
        linux-media@...r.kernel.org, linux-staging@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel@...i.sm, phone-devel@...r.kernel.org
Subject: Re: [PATCHv3 1/1 RESEND] media: imx: Round line size to 4 bytes

Hello,

On Thu, 24 Nov 2022 13:30:15 +0200
Laurent Pinchart <laurent.pinchart@...asonboard.com> wrote:

> Hi Dorota,
> 
> On Tue, Nov 01, 2022 at 01:38:34PM +0100, Dorota Czaplejewicz wrote:
> > On Mon, 17 Oct 2022 16:30:59 +0200 Dorota Czaplejewicz wrote:  
> > > On Mon, 17 Oct 2022 11:28:57 +0300 Laurent Pinchart wrote:  
> > 
> > [snip]
> >   
> > > > I would also very much appreciate feedback from NXP on this. There's a
> > > > risk of ill side-effects that I would prefer ruling out very clearly.    
> > > 
> > > I posted a question on the NXP forum: https://community.nxp.com/t5/i-MX-Processors/i-MX8MQ-CSI-line-size-constraint-documentation-mistake/m-p/1538629#M196448
> > >   
> > 
> > This question received a reply from NXP today: https://community.nxp.com/t5/i-MX-Processors/i-MX8MQ-CSI-line-size-constraint-documentation-mistake/m-p/1546872/highlight/true#M197067
> > 
> > Quoting NXP's answer:
> >   
> > > Do I understand correctly, that streams divisible by 4×4 will work on other i.MX8 hardware too? - Yes
> > > Will those kind of resolutions work on i.MX7 series CSI hardware? - Yes  
> > 
> > which implies no bad side effects the way I read it. Is this
> > sufficient for the kernel?  
> 
> Ideally I'd like to test that on i.MX7 but I don't think I'll have time
> to do so in the near future. I don't want to block the patch on this, so
> I'm fine merging it, but I'd like a comment in the code that explains
> why it's safe to depart from the documentation.
> 
> Also, the discussion on the NXP forum said that a width that is a
> multiple of 4 bytes but not a multiple of 8 bytes works only if the
> height is also a multiple of 4. I don't see that constraint being
> enforced in the code, am I missing something ?

Thanks for pointing it out, I'll have to double-check it.
> 
> You mentioned in the forum that you tried 1052x779 and that failed. How
> did it fail ? Have you also tried 1052x778 ?

I tried with many resolutions when I was checking out the sensor, and came to the conclusion that the line count must be even. Otherwise the last 4 bytes of the buffer would end up corrupted.

My guess is that the copy engine can indeed only do bursts of 8 bytes, but that the whole buffer is copied at once rather than line-by-line.
> 
> > > > > Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz@...i.sm>
> > > > > ---
> > > > > Hello,
> > > > > 
> > > > > the Librem 5 is using an out-of-tree driver for s5k3l6xx, and
> > > > > rounding to 4 is optimal to operate it.
> > > > > 
> > > > > This revision improves the commit message.
> > > > > 
> > > > > Cheers,
> > > > > Dorota Czaplejewicz
> > > > > 
> > > > >  drivers/staging/media/imx/imx7-media-csi.c | 4 ++--
> > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/staging/media/imx/imx7-media-csi.c b/drivers/staging/media/imx/imx7-media-csi.c
> > > > > index a0553c24cce4..af821b410c3f 100644
> > > > > --- a/drivers/staging/media/imx/imx7-media-csi.c
> > > > > +++ b/drivers/staging/media/imx/imx7-media-csi.c
> > > > > @@ -999,10 +999,10 @@ static int imx7_csi_mbus_fmt_to_pix_fmt(struct v4l2_pix_format *pix,
> > > > >  	}
> > > > >  
> > > > >  	/* Round up width for minimum burst size */
> > > > > -	width = round_up(mbus->width, 8);
> > > > > +	width = round_up(mbus->width, 4);
> > > > >  
> > > > >  	/* Round up stride for IDMAC line start address alignment */
> > > > > -	stride = round_up((width * cc->bpp) >> 3, 8);
> > > > > +	stride = round_up((width * cc->bpp) >> 3, 4);
> > > > >  
> > > > >  	pix->width = width;
> > > > >  	pix->height = mbus->height;      
> 


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