[PATCH] drm/vc4: Fix HDMI mode validation

[Nicolas Saenz Julienne]

Current mode validation impedes setting up some video modes which should
be supported otherwise. Namely 1920x1200@60Hz.

Fix this by lowering the minimum HDMI state machine clock to pixel clock
ratio allowed.

Fixes: 32e823c63e90 ("drm/vc4: Reject HDMI modes with too high of clocks")
Reported-by: Stefan Wahren <stefan.wahren@xxxxxxxx>
Suggested-by: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
---
 drivers/gpu/drm/vc4/vc4_hdmi.c | 20 ++++++++++++++++----
 1 file 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;
-- 
2.25.1