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-rw-r--r--drivers/gpu/drm/vc4/vc4_hdmi.c20
1 files changed, 16 insertions, 4 deletions
diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
index cea18dc15f77..340719238753 100644
--- a/drivers/gpu/drm/vc4/vc4_hdmi.c
+++ b/drivers/gpu/drm/vc4/vc4_hdmi.c
@@ -681,11 +681,23 @@ static enum drm_mode_status
vc4_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.
+ /*
+ * As stated in RPi's vc4 firmware "HDMI state machine (HSM) clock must
+ * be faster than pixel clock, infinitesimally faster, tested in
+ * simulation. Otherwise, exact value is unimportant for HDMI
+ * operation." This conflicts with bcm2835's vc4 documentation, which
+ * states HSM's clock has to be at least 108% of the pixel clock.
+ *
+ * Real life tests reveal that vc4's firmware statement holds up, and
+ * users are able to use pixel clocks closer to HSM's, namely for
+ * 1920x1200@60Hz. So it was decided to have leave a 1% margin between
+ * both clocks. Which, for RPi0-3 implies a maximum pixel clock of
+ * 162MHz.
+ *
+ * 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))
+ if (mode->clock > HSM_CLOCK_FREQ / (1000 * 101 / 100))
return MODE_CLOCK_HIGH;
return MODE_OK;