drm/vc4: Fix HDMI mode validation

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@i2se.com>
Suggested-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200326122001.22215-1-nsaenzjulienne@suse.de

diff --git a/drivers/gpu/drm/vc4/vc4_hdmi.c b/drivers/gpu/drm/vc4/vc4_hdmi.c
index cea18dc..3407192 100644 (file)
--- 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;