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Date:   Wed, 13 Oct 2021 11:26:36 +0200
From:   Dorota Czaplejewicz <dorota.czaplejewicz@...i.sm>
To:     Philipp Zabel <p.zabel@...gutronix.de>
Cc:     Steve Longerbeam <slongerbeam@...il.com>,
        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: [PATCH] media: imx: Round line size to 4 bytes

Hello,

On Fri, 08 Oct 2021 14:19:41 +0200
Philipp Zabel <p.zabel@...gutronix.de> wrote:

> Hi Dorota,
> 
> On Wed, 2021-10-06 at 13:05 +0200, Dorota Czaplejewicz wrote:
> > Section 13.7.6.13 "CSI Image Parameter Register" of the
> > i.MX 8M Quad Applications Processors Reference Manual
> > states that the line size should be divisible by 8 bytes.
> > However, the hardware also accepts sizes divisible by 4 bytes.
> > 
> > This patch accepts line sizes divisible 4-bytes in non-planar mode.  
> 
> Thank you, this makes it much clearer. I see two issues with this,
> though, one small and one a bit bigger:
> 
> First, I'd be wary of disregarding the reference manual - unless we know
> better, and then it should be well documented in the code. It might be
> that the 8-byte alignment requirement stems from the fact that the FIFO
> operates in double-word units, which might cause the CSI to write over
> the end of the buffer if the line width is odd (in 32-bit words).
> Or maybe it's just that the FBUF_STRIDE conflicts with this, I'm unclear
> on whether that is only given in units of dwords (although the driver
> currently doesn't support this anyway).
> 
> I wonder: if you use 4-byte aligned width and odd height, does the CSI
> write over the end of the buffer?

I tested this case, and found a glitch which suggests the last 4 bytes are ignored:

https://source.puri.sm/Librem5/linux-next/uploads/cfb59e3832431aaa3a69549455502568/image.png

That would be taken care of rounding up towards a number decided at runtime, like:

divisor = 8 >> (mbus->height % 2);
> 
> > Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz@...i.sm>
> > ---
> > 
> > Hello,
> > 
> > my previous patch identified something that was not a problem,
> > so I'm sending a different one.
> > 
> > This has been tested on the Librem 5.
> > 
> > Cheers,
> > Dorota
> > 
> >  drivers/staging/media/imx/imx-media-utils.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/staging/media/imx/imx-media-utils.c b/drivers/staging/media/imx/imx-media-utils.c
> > index 5128915a5d6f..a303003929e3 100644
> > --- a/drivers/staging/media/imx/imx-media-utils.c
> > +++ b/drivers/staging/media/imx/imx-media-utils.c
> > @@ -545,13 +545,13 @@ int imx_media_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 */
> >  	if (cc->planar)
> >  		stride = round_up(width, 16);
> >  	else
> > -		stride = round_up((width * cc->bpp) >> 3, 8);
> > +		stride = round_up((width * cc->bpp) >> 3, 4);  
> 
> Second, even if this works fine on the i.MX7/8M CSI, the alignment is
> still required for the i.MX5/6 IPU, for which this code and the comments
> were written. So we need a way to differentiate the two cases here.
> 
> regards
> Philipp

How best to go about this? I can see in the file imx-media-capture.c that there the video device lives in struct capture_priv.vdev.vfd. Would that be the right place to query about the underlying hardware?

Then the following functions would gain a new "small_divisor" parameter:
- imx_media_mbus_fmt_to_pix_fmt (a GPL symbol)
- imx_media_mbus_fmt_to_ipu_image (a GPL symbol)
- __capture_try_fmt

Those would have to extract the device type from struct capture_priv:
- __capture_legacy_try_fmt
- capture_try_fmt_vid_cap
- capture_s_fmt_vid_cap
- capture_init_format

Regards,
Dorota

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