Peter Robinson reported issues on Fedora with 4k monitors not having
their modes filtered down to 1920x1080 on Raspberry Pi.  Hopefully
this resolves that.

Cc: Peter Robinson <pbrobin...@redhat.com>
Signed-off-by: Eric Anholt <e...@anholt.net>
---

Note: This is an untested patch, since I'm away from hardware
currently.

 drivers/gpu/drm/vc4/vc4_hdmi.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
index fa37a1c07cf6..65826a4f208e 100644
--- a/drivers/gpu/drm/vc4/vc4_hdmi.c
+++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
@@ -694,7 +694,22 @@ static void vc4_hdmi_encoder_enable(struct drm_encoder 
*encoder)
        }
 }
 
+static enum drm_mode_status
+vc5_hdmi_encoder_mode_valid(struct drm_encoder *crtc,
+                           const struct drm_display_mode *mode)
+{
+       /* HSM clock must be 108% of the pixel clock.  Additionally,
+        * the AXI clock needs to be at least 25% of pixel clock, but
+        * HSM ends up being the limiting factor.
+        */
+       if (mode->clock > HSM_CLOCK_FREQ / (1000 * 108 / 100))
+               return MODE_CLOCK_HIGH;
+
+       return MODE_OK;
+}
+
 static const struct drm_encoder_helper_funcs vc4_hdmi_encoder_helper_funcs = {
+       .mode_valid = vc5_hdmi_encoder_mode_valid,
        .disable = vc4_hdmi_encoder_disable,
        .enable = vc4_hdmi_encoder_enable,
 };
-- 
2.14.1

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