linux-kernel - [PATCH] drm/meson: vclk: fix calculation of 59.94 fractional rates

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Date: Tue,  9 Jan 2024 23:07:04 +0000
From: Christian Hewitt <christianshewitt@...il.com>
To: Neil Armstrong <neil.armstrong@...aro.org>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>,
	Daniel Vetter <daniel@...ll.ch>,
	Kevin Hilman <khilman@...libre.com>,
	Jerome Brunet <jbrunet@...libre.com>,
	Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
	dri-devel@...ts.freedesktop.org,
	linux-amlogic@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Cc: Christian Hewitt <christianshewitt@...il.com>
Subject: [PATCH] drm/meson: vclk: fix calculation of 59.94 fractional rates

Playing 4K media with 59.94 fractional rate (typically VP9) causes the screen to lose
sync with the following error reported in the system log:

[   89.610280] Fatal Error, invalid HDMI vclk freq 593406

Modetest shows the following:

3840x2160 59.94 3840 4016 4104 4400 2160 2168 2178 2250 593407 flags: xxxx, xxxx,
drm calculated value -------------------------------------^

Change the fractional rate calculation to stop DIV_ROUND_CLOSEST rounding down which
results in vclk freq failing to match correctly.

Fixes: e5fab2ec9ca4 ("drm/meson: vclk: add support for YUV420 setup")
Signed-off-by: Christian Hewitt <christianshewitt@...il.com>
---
I'm unable to give a better mathematical description of the fix as I can barely read
code. The change was inspired by [0] which I chanced upon while looking at how other
dw-hdmi drivers handle fractional rates.

[0] https://github.com/torvalds/linux/commit/4f510aa10468954b1da4e94689c38ac6ea8d3627

 drivers/gpu/drm/meson/meson_vclk.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/meson/meson_vclk.c b/drivers/gpu/drm/meson/meson_vclk.c
index 2a82119eb58e..2a942dc6a6dc 100644
--- a/drivers/gpu/drm/meson/meson_vclk.c
+++ b/drivers/gpu/drm/meson/meson_vclk.c
@@ -790,13 +790,13 @@ meson_vclk_vic_supported_freq(struct meson_drm *priv, unsigned int phy_freq,
 				 FREQ_1000_1001(params[i].pixel_freq));
 		DRM_DEBUG_DRIVER("i = %d phy_freq = %d alt = %d\n",
 				 i, params[i].phy_freq,
-				 FREQ_1000_1001(params[i].phy_freq/10)*10);
+				 FREQ_1000_1001(params[i].phy_freq/1000)*1000);
 		/* Match strict frequency */
 		if (phy_freq == params[i].phy_freq &&
 		    vclk_freq == params[i].vclk_freq)
 			return MODE_OK;
 		/* Match 1000/1001 variant */
-		if (phy_freq == (FREQ_1000_1001(params[i].phy_freq/10)*10) &&
+		if (phy_freq == (FREQ_1000_1001(params[i].phy_freq/1000)*1000) &&
 		    vclk_freq == FREQ_1000_1001(params[i].vclk_freq))
 			return MODE_OK;
 	}
@@ -1070,7 +1070,7 @@ void meson_vclk_setup(struct meson_drm *priv, unsigned int target,
 
 	for (freq = 0 ; params[freq].pixel_freq ; ++freq) {
 		if ((phy_freq == params[freq].phy_freq ||
-		     phy_freq == FREQ_1000_1001(params[freq].phy_freq/10)*10) &&
+		     phy_freq == FREQ_1000_1001(params[freq].phy_freq/1000)*1000) &&
 		    (vclk_freq == params[freq].vclk_freq ||
 		     vclk_freq == FREQ_1000_1001(params[freq].vclk_freq))) {
 			if (vclk_freq != params[freq].vclk_freq)
-- 
2.34.1