[PATCH] media: imx: Fix rounding

[Dorota Czaplejewicz]

Hi,

On Fri, 24 Sep 2021 13:46:15 +0200
Philipp Zabel <p.zabel at pengutronix.de> wrote:

> Hi Dorota,
> 
> On Fri, Sep 24, 2021 at 12:06:31PM +0200, Dorota Czaplejewicz wrote:
> > Change rounding to the minimal burst size from 2^n to n.
> > 
> > This fixes images with sizes that are a multiple of 8 pixels.  
> 
> Could you elaborate on what is currently wrong with images that are a
> multiple of 8 pixels wide? Or are you rather trying to add support
> for images that are not a multiple of 8 pixels wide?
> 
> > 
> > See section 13.7.6.13 CSI Image Parameter Register of the
> > i.MX 8M Quad Applications Processors Reference Manual.
> > 
> > Fixes: 451a7b7815d0b ("media: imx: lift CSI and PRP ENC/VF width
> > alignment restriction")
> > Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz at puri.sm>
> > ---
> > Hi,
> > 
> > I tested this patch on the Librem 5 with the main camera.
> > 
> > --Dorota
> >  drivers/staging/media/imx/imx-media-utils.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/staging/media/imx/imx-media-utils.c b/drivers/staging/media/imx/imx-media-utils.c
> > index 5128915a5d6f..a2d8fab32a39 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, 3);  
> 
> That is not a valid use of the round_up() macro anymore.
> The second parameter must be a power of 2.
> 
> regards
> Philipp

Thanks for catching that. It turns out I was fixing an image that was a multiple of 4, but not a multiple of 8.

The problem that occurred has to do with an image of size 1052x780, which covers the entire sensor in use at 1/4 scale.

The sensor was configured to emit 1052*780 8-bit pixels, whereas the CSI portion here rounded up the image, expecting 1056*780. As a result, the userspace received a buffer sized according to the latter, each frame padded with some pixels from the next. As a result, each frame was composed of two or more, with a broken TV effect.

The data sheet says that the width parameter "should" be a multiple of 8:

> If the input data from the sensor is 8-bit/pixel format, the IMAGE_WIDTH should be a multiple of 8 pixels.

Nevertheless, I double-tested this, and changing the relevant rounding from "8" to "4" works flawlessly here. Lower divisors were tested previously and did not work.

Does it make sense to accept 4 as a valid divisor, given that rounding is likely not going to help but result in an invalid image anyway (as it did for me)?

Cheers,
Dorota
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