[PATCH V2 2/2] drm/bridge: samsung-dsim: Fix porch calcalcuation rounding

[Adam Ford]

On Thu, Apr 25, 2024 at 4:19 AM Marek Szyprowski
<m.szyprowski at samsung.com> wrote:
>
> On 12.02.2024 00:09, Adam Ford wrote:
> > When using video sync pulses, the HFP, HBP, and HSA are divided between
> > the available lanes if there is more than one lane.  For certain
> > timings and lane configurations, the HFP may not be evenly divisible.
> > If the HFP is rounded down, it ends up being too small which can cause
> > some monitors to not sync properly. In these instances, adjust htotal
> > and hsync to round the HFP up, and recalculate the htotal.
> >
> > Tested-by: Frieder Schrempf <frieder.schrempf at kontron.de> # Kontron BL i.MX8MM with HDMI monitor
> > Signed-off-by: Adam Ford <aford173 at gmail.com>
>
> Tested-by: Marek Szyprowski <m.szyprowski at samsung.com>

Thank you very much for testing!

>
> > ---
> > V2:  No changes
> >
> > diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> > index 8476650c477c..52939211fe93 100644
> > --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> > +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> > @@ -1606,6 +1606,27 @@ static int samsung_dsim_atomic_check(struct drm_bridge *bridge,
> >               adjusted_mode->flags |= (DRM_MODE_FLAG_PHSYNC | DRM_MODE_FLAG_PVSYNC);
> >       }
> >
> > +     /*
> > +      * When using video sync pulses, the HFP, HBP, and HSA are divided between
> > +      * the available lanes if there is more than one lane.  For certain
> > +      * timings and lane configurations, the HFP may not be evenly divisible.
> > +      * If the HFP is rounded down, it ends up being too small which can cause
> > +      * some monitors to not sync properly. In these instances, adjust htotal
> > +      * and hsync to round the HFP up, and recalculate the htotal. Through trial
> > +      * and error, it appears that the HBP and HSA do not appearto need the same
> > +      * correction that HFP does.
> > +      */
> > +     if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO_SYNC_PULSE && dsi->lanes > 1) {

Frieder  &  Marek S,

Marek V is proposing we eliminate the check against the flags and do
it unconditionally.  If I send you both a different patch, would you
be willing to try them on your platforms?  I don't want to risk
breaking a board.
I used the check above from the NXP downstream kernel, so it felt
safe, but I am not as familiar with the different DSI modes, so I am
not sure what the impact would be if this read:

 if (dsi->lanes > 1) {

Does anyone else have an opinion on this?
> > +             int hfp = adjusted_mode->hsync_start - adjusted_mode->hdisplay;
> > +             int remainder = hfp % dsi->lanes;
> > +
> > +             if (remainder) {
> > +                     adjusted_mode->hsync_start += remainder;
> > +                     adjusted_mode->hsync_end   += remainder;
> > +                     adjusted_mode->htotal      += remainder;
> > +             }
> > +     }
> > +
> >       return 0;
> >   }
> >
>
> Best regards
> --
> Marek Szyprowski, PhD
> Samsung R&D Institute Poland
>