drm/edid: Make the detailed timing CEA/HDMI mode fixup accept up to 5kHz clock difference

Rather than using drm_match_cea_mode() to see if the EDID detailed
timings are supposed to represent one of the CEA/HDMI modes, add a
special version of that function that takes in an explicit clock
tolerance value (in kHz). When looking at the detailed timings specify
the tolerance as 5kHz due to the 10kHz clock resolution limit inherent
in detailed timings.

drm_match_cea_mode() uses the normal KHZ2PICOS() matching of clocks,
which only allows smaller errors for lower clocks (eg. for 25200 it
won't allow any error) and a bigger error for higher clocks (eg. for
297000 it actually matches 296913-297000). So it doesn't really match
what we want for the fixup. Using the explicit +-5kHz is much better
for this use case.

Not sure if we should change the normal mode matching to also use
something else besides KHZ2PICOS() since it allows a different
proportion of error depending on the clock. I believe VESA CVT
allows a maximum deviation of .5%, so using that for normal mode
matching might be a good idea?

Cc: Adam Jackson <ajax@redhat.com>
Tested-by: nathan.d.ciobanu@linux.intel.com
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92217
Fixes: fa3a7340eaa1 ("drm/edid: Fix up clock for CEA/HDMI modes specified via detailed timings")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 60 insertions, 2 deletions

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index d5d2c03fd136..c214f1246cb4 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -2545,6 +2545,33 @@ cea_mode_alternate_clock(const struct drm_display_mode *cea_mode)
 	return clock;
 }
 
+static u8 drm_match_cea_mode_clock_tolerance(const struct drm_display_mode *to_match,
+					     unsigned int clock_tolerance)
+{
+	u8 mode;
+
+	if (!to_match->clock)
+		return 0;
+
+	for (mode = 0; mode < ARRAY_SIZE(edid_cea_modes); mode++) {
+		const struct drm_display_mode *cea_mode = &edid_cea_modes[mode];
+		unsigned int clock1, clock2;
+
+		/* Check both 60Hz and 59.94Hz */
+		clock1 = cea_mode->clock;
+		clock2 = cea_mode_alternate_clock(cea_mode);
+
+		if (abs(to_match->clock - clock1) > clock_tolerance &&
+		    abs(to_match->clock - clock2) > clock_tolerance)
+			continue;
+
+		if (drm_mode_equal_no_clocks(to_match, cea_mode))
+			return mode + 1;
+	}
+
+	return 0;
+}
+
 /**
  * drm_match_cea_mode - look for a CEA mode matching given mode
  * @to_match: display mode
@@ -2609,6 +2636,33 @@ hdmi_mode_alternate_clock(const struct drm_display_mode *hdmi_mode)
 	return cea_mode_alternate_clock(hdmi_mode);
 }
 
+static u8 drm_match_hdmi_mode_clock_tolerance(const struct drm_display_mode *to_match,
+					      unsigned int clock_tolerance)
+{
+	u8 mode;
+
+	if (!to_match->clock)
+		return 0;
+
+	for (mode = 0; mode < ARRAY_SIZE(edid_4k_modes); mode++) {
+		const struct drm_display_mode *hdmi_mode = &edid_4k_modes[mode];
+		unsigned int clock1, clock2;
+
+		/* Make sure to also match alternate clocks */
+		clock1 = hdmi_mode->clock;
+		clock2 = hdmi_mode_alternate_clock(hdmi_mode);
+
+		if (abs(to_match->clock - clock1) > clock_tolerance &&
+		    abs(to_match->clock - clock2) > clock_tolerance)
+			continue;
+
+		if (drm_mode_equal_no_clocks(to_match, hdmi_mode))
+			return mode + 1;
+	}
+
+	return 0;
+}
+
 /*
  * drm_match_hdmi_mode - look for a HDMI mode matching given mode
  * @to_match: display mode
@@ -3119,14 +3173,18 @@ static void fixup_detailed_cea_mode_clock(struct drm_display_mode *mode)
 	u8 mode_idx;
 	const char *type;
 
-	mode_idx = drm_match_cea_mode(mode) - 1;
+	/*
+	 * allow 5kHz clock difference either way to account for
+	 * the 10kHz clock resolution limit of detailed timings.
+	 */
+	mode_idx = drm_match_cea_mode_clock_tolerance(mode, 5) - 1;
 	if (mode_idx < ARRAY_SIZE(edid_cea_modes)) {
 		type = "CEA";
 		cea_mode = &edid_cea_modes[mode_idx];
 		clock1 = cea_mode->clock;
 		clock2 = cea_mode_alternate_clock(cea_mode);
 	} else {
-		mode_idx = drm_match_hdmi_mode(mode) - 1;
+		mode_idx = drm_match_hdmi_mode_clock_tolerance(mode, 5) - 1;
 		if (mode_idx < ARRAY_SIZE(edid_4k_modes)) {
 			type = "HDMI";
 			cea_mode = &edid_4k_modes[mode_idx];