Re: [PATCH] drm/msm/dpu: don't try using 2 LMs if only one DSC is available

[Dmitry Baryshkov]

On Tue, Mar 17, 2026 at 05:48:59PM +0100, Konrad Dybcio wrote:
> On 3/17/26 4:30 PM, Dmitry Baryshkov wrote:
> > Current topology code will try using 2 LMs with just one DSC, which
> > breaks cases like SC7280 / Fairphone5.
> 
> (why/how?)

See https://salsa.debian.org/Mobian-team/devices/kernels/qcom-linux/-/issues/41

Using 2 LM/2PP + 3D + 1DSC is a separate topology. I'm checking what
would be required to fix it. For now this is the easiest fix to forbid
using it (it wasn't supported beforehand anyway and got enabled by the
encoder / CRTC refactoring).

> 
> Konrad
> 
> >  Forbid using 2 LMs split in such
> > a case.
> > 
> > Fixes: 1ce69c265a53 ("drm/msm/dpu: move resource allocation to CRTC")
> > Reported-by: Luca Weiss <luca.weiss@xxxxxxxxxxxxx>
> > Closes: https://lore.kernel.org/r/DH1IKLU0YZYU.2SW4WYO7H3H4R@xxxxxxxxxxxxx/
> > Tested-by: Luca Weiss <luca.weiss@xxxxxxxxxxxxx> # qcm6490-fairphone-fp5
> > Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c
> > index 6bf7c46379ae..96b67ea3fb4c 100644
> > --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c
> > +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_crtc.c
> > @@ -1410,7 +1410,8 @@ static struct msm_display_topology dpu_crtc_get_topology(
> >  		topology.num_lm = 2;
> >  	else if (topology.num_dsc == 2)
> >  		topology.num_lm = 2;
> > -	else if (dpu_kms->catalog->caps->has_3d_merge)
> > +	else if (dpu_kms->catalog->caps->has_3d_merge &&
> > +		 topology.num_dsc == 0)
> >  		topology.num_lm = (mode->hdisplay > MAX_HDISPLAY_SPLIT) ? 2 : 1;
> >  	else
> >  		topology.num_lm = 1;
> > 
> > ---
> > base-commit: 50c4a49f7292b33b454ea1a16c4f77d6965405dc
> > change-id: 20260317-fix-3d-dsc-4b073977ea19
> > 
> > Best regards,

-- 
With best wishes
Dmitry