Re: [PATCH 4/5] drm/msm/dpu1: Account for DSC's bits_per_pixel having 4 fractional bits

From: Marijn Suijten
Date: Tue Oct 04 2022 - 18:12:14 EST

On 2022-10-04 10:03:07, Abhinav Kumar wrote:
> On 10/1/2022 12:08 PM, Marijn Suijten wrote:
> > According to the comment this DPU register contains the bits per pixel
> > as a 6.4 fractional value, conveniently matching the contents of
> > bits_per_pixel in struct drm_dsc_config which also uses 4 fractional
> > bits. However, the downstream source this implementation was
> > copy-pasted from has its bpp field stored _without_ fractional part.
> >
> > This makes the entire convoluted math obsolete as it is impossible to
> > pull those 4 fractional bits out of thin air, by somehow trying to reuse
> > the lowest 2 bits of a non-fractional bpp (lsb = bpp % 4??).
> >
> > The rest of the code merely attempts to keep the integer part a multiple
> > of 4, which is rendered useless thanks to data |= dsc->bits_per_pixel <<
> > 12; already filling up those bits anyway (but not on downstream).
> >
> > Fixes: c110cfd1753e ("drm/msm/disp/dpu1: Add support for DSC")
> > Signed-off-by: Marijn Suijten <marijn.suijten@xxxxxxxxxxxxxx>
> Many of this bugs are because the downstream code from which this
> implementation was derived wasnt the latest perhaps?

Perhaps, this code is "identical" to what I'm looking at in some
downstream 4.14 / 4.19, where the upstream struct for DSC either wasn't
there or wasn't used. We have to find and address these bugs one by one
to make our panels work, and this series gets one platform (sdm845) down
but has more work pending for others (sm8250 has my current focus).

Or are you suggesting to "redo" the DSC integration work based on a
(much) newer display techpack (SDE driver)?

> Earlier, downstream had its own DSC struct maybe leading to this
> redundant math but now we have migrated over to use the upstream struct
> drm_dsc_config.

Found the 3-year-old `disp: msm: use upstream dsc config data` commit
that makes this change. It carries a similar comment:

/* integer bpp support only */

The superfluous math was howerver removed earlier, in:

disp: msm: fix dsc parameters related to 10 bpc 10 bpp

- Marijn

> That being said, this patch LGTM
> Reviewed-by: Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx>