[PATCH] drm/amdgpu: Respect max pixel clock for HDMI and DVI-D

[Alex Deucher]

On Wed, Aug 27, 2025 at 10:36 AM Timur Kristóf <timur.kristof at gmail.com> wrote:
>
> On Tue, 2025-08-26 at 10:03 -0400, Alex Deucher wrote:
> > On Mon, Aug 25, 2025 at 6:10 PM Timur Kristóf
> > <timur.kristof at gmail.com> wrote:
> > >
> > > Change the legacy (non-DC) display code to respect the maximum
> > > pixel clock for HDMI and DVI-D. Reject modes that would require
> > > a higher pixel clock than can be supported.
> > >
> > > Also update the maximum supported HDMI clock value depending on
> > > the ASIC type.
> > >
> > > For reference, see the DC code:
> > > check max_hdmi_pixel_clock in dce*_resource.c
> > >
> > > Signed-off-by: Timur Kristóf <timur.kristof at gmail.com>
> > > ---
> > >  .../gpu/drm/amd/amdgpu/amdgpu_connectors.c    | 21
> > > +++++++++++++++++--
> > >  1 file changed, 19 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
> > > b/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
> > > index 5e375e9c4f5d..abcc4469cf57 100644
> > > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
> > > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_connectors.c
> > > @@ -1195,12 +1195,26 @@ static void
> > > amdgpu_connector_dvi_force(struct drm_connector *connector)
> > >                 amdgpu_connector->use_digital = true;
> > >  }
> > >
> > > +/**
> > > + * Returns the maximum supported HDMI (TMDS) pixel clock in KHz.
> > > + */
> > > +static int amdgpu_max_hdmi_pixel_clock(const struct amdgpu_device
> > > *adev)
> > > +{
> > > +       if (adev->asic_type >= CHIP_POLARIS10)
> > > +               return 600000;
> > > +       else if (adev->asic_type >= CHIP_TONGA)
> > > +               return 300000;
> > > +       else
> > > +               return 297000;
> > > +}
> > > +
> > >  static enum drm_mode_status amdgpu_connector_dvi_mode_valid(struct
> > > drm_connector *connector,
> > >                                             const struct
> > > drm_display_mode *mode)
> > >  {
> > >         struct drm_device *dev = connector->dev;
> > >         struct amdgpu_device *adev = drm_to_adev(dev);
> > >         struct amdgpu_connector *amdgpu_connector =
> > > to_amdgpu_connector(connector);
> > > +       const int max_hdmi_pixel_clock =
> > > amdgpu_max_hdmi_pixel_clock(adev);
> > >
> > >         /* XXX check mode bandwidth */
> > >
> > > @@ -1208,10 +1222,13 @@ static enum drm_mode_status
> > > amdgpu_connector_dvi_mode_valid(struct drm_connector
> > >                 if ((amdgpu_connector->connector_object_id ==
> > > CONNECTOR_OBJECT_ID_DUAL_LINK_DVI_I) ||
> > >                     (amdgpu_connector->connector_object_id ==
> > > CONNECTOR_OBJECT_ID_DUAL_LINK_DVI_D) ||
> > >                     (amdgpu_connector->connector_object_id ==
> > > CONNECTOR_OBJECT_ID_HDMI_TYPE_B)) {
> > > -                       return MODE_OK;
> > > +                       if (mode->clock > max_hdmi_pixel_clock)
> > > +                               return MODE_CLOCK_HIGH;
> > > +                       else
> > > +                               return MODE_OK;
> >
> > I don't think it makes sense to limit dual link DVI to the max HDMI
> > clock.  HDMI is single link TMDS.  Other than that, the patch looks
> > good to me.
> >
> > Alex
>
> Hi Alex,
>
> The problem I'm trying to solve here is that on the non-DC code, by
> default I get a black screen on DVI-D because the code doesn't set an
> upper limit on what is actually supported. (And the monitor supports
> some modes which won't work.)
>
> As far as I see, DC only allows 4K 30 Hz on DVI for the same connector
> and DVI/HDMI adapter. That may be wrong, though.
>
> What value do you suggest to use?
>
> If HDMI = single link TMDS, does that mean that a passive DVI/HDMI
> adapter cannot take advantage of dual-link DVI?

DVI has a 165 Mhz limit per link so dual link would be 330 Mhz.

Alex

>
> Thanks,
> Timur
>
> >
> > >                 } else if (connector->display_info.is_hdmi) {
> > >                         /* HDMI 1.3+ supports max clock of 340 Mhz
> > > */
> > > -                       if (mode->clock > 340000)
> > > +                       if (mode->clock > max_hdmi_pixel_clock)
> > >                                 return MODE_CLOCK_HIGH;
> > >                         else
> > >                                 return MODE_OK;
> > > --
> > > 2.50.1
> > >