[PATCH] [RFC] drm/stm: Prefer faster display pixel clock over slower ones

[Marek Vasut]

If the delta between requested pixelclock and resulting pixelclock
is larger than the delta between requested pixelclock and the next
step in available pixelclock (limited to 10% of pixelclock to avoid
too much out-of-specification operation), use the faster pixelclock.

This fixes the condition where the resulting pixelclock is much
slower than the lowest clock rate supported by the display, while
the next available pixelclock are just slightly faster than the
highest clock rate supported by the display. Using the lower clock
rate leads e.g. to subtle artifacts barely visible on the display,
like flickering pixels. Using slightly faster clock leads to no
such effect.

Signed-off-by: Marek Vasut <marex at denx.de>
Cc: Alexandre Torgue <alexandre.torgue at st.com>
Cc: Benjamin Gaignard <benjamin.gaignard at st.com>
Cc: Maxime Coquelin <mcoquelin.stm32 at gmail.com>
Cc: Philippe Cornu <philippe.cornu at st.com>
Cc: Vincent Abriou <vincent.abriou at st.com>
Cc: Yannick Fertré <yannick.fertre at st.com>
Cc: dri-devel at lists.freedesktop.org
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-stm32 at st-md-mailman.stormreply.com
---
 drivers/gpu/drm/stm/ltdc.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/stm/ltdc.c b/drivers/gpu/drm/stm/ltdc.c
index 6e28f707092f..07c73079293c 100644
--- a/drivers/gpu/drm/stm/ltdc.c
+++ b/drivers/gpu/drm/stm/ltdc.c
@@ -507,6 +507,25 @@ static bool ltdc_crtc_mode_fixup(struct drm_crtc *crtc,
 {
 	struct ltdc_device *ldev = crtc_to_ltdc(crtc);
 	int rate = mode->clock * 1000;
+	int rate_min = clk_round_rate(ldev->pixel_clk, rate);
+	int rate_max = clk_round_rate(ldev->pixel_clk, rate + (rate / 10));
+
+	/*
+	 * If the delta between requested pixelclock and resulting pixelclock
+	 * is larger than the delta between requested pixelclock and the next
+	 * step in available pixelclock (limited to 10% of pixelclock to avoid
+	 * too much out-of-specification operation), use the faster pixelclock.
+	 *
+	 * This fixes the condition where the resulting pixelclock is much
+	 * slower than the lowest clock rate supported by the display, while
+	 * the next available pixelclock are just slightly faster than the
+	 * highest clock rate supported by the display. Using the lower clock
+	 * rate leads e.g. to subtle artifacts barely visible on the display,
+	 * like flickering pixels. Using slightly faster clock leads to no
+	 * such effect.
+	 */
+	if (rate - rate_min > rate_max - rate)
+		rate = rate_max;
 
 	if (clk_set_rate(ldev->pixel_clk, rate) < 0) {
 		DRM_ERROR("Cannot set rate (%dHz) for pixel clk\n", rate);
-- 
2.28.0