[PATCH] drm/bridge: tc358767: Improve DPI output pixel clock accuracy

[Marek Vasut]

The Pixel PLL is not very capable and may come up with wildly inaccurate
clock. Since DPI panels are often tolerant to slightly higher pixel clock
without being operated outside of specification, calculate two Pixel PLL
settings for DPI output, one for desired output pixel clock and one for
output pixel clock with 1% increase, and then pick the result which is
closer to the desired pixel clock and use it as the Pixel PLL settings.

For the Chefree CH101 panel with 13 MHz Xtal input clock, the frequency
without this patch is 65 MHz which is out of the panel specification of
68.9..73.4 MHz, while with this patch it is 71.5 MHz which is well within
the specification and far more accurate.

Signed-off-by: Marek Vasut <marex at denx.de>
---
Cc: Andrzej Hajda <andrzej.hajda at intel.com>
Cc: David Airlie <airlied at gmail.com>
Cc: Jernej Skrabec <jernej.skrabec at gmail.com>
Cc: Jonas Karlman <jonas at kwiboo.se>
Cc: Laurent Pinchart <Laurent.pinchart at ideasonboard.com>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Cc: Maxime Ripard <mripard at kernel.org>
Cc: Neil Armstrong <neil.armstrong at linaro.org>
Cc: Robert Foss <rfoss at kernel.org>
Cc: Simona Vetter <simona at ffwll.ch>
Cc: Thomas Zimmermann <tzimmermann at suse.de>
Cc: dri-devel at lists.freedesktop.org
---
 drivers/gpu/drm/bridge/tc358767.c | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/bridge/tc358767.c b/drivers/gpu/drm/bridge/tc358767.c
index 0d523322fdd8e..7e1a7322cec70 100644
--- a/drivers/gpu/drm/bridge/tc358767.c
+++ b/drivers/gpu/drm/bridge/tc358767.c
@@ -1682,15 +1682,39 @@ static int tc_dpi_atomic_check(struct drm_bridge *bridge,
 			       struct drm_connector_state *conn_state)
 {
 	struct tc_data *tc = bridge_to_tc(bridge);
-	int adjusted_clock = 0;
+	int adjusted_clock_0p = 0;
+	int adjusted_clock_1p = 0;
+	int adjusted_clock;
 	int ret;
 
+	/*
+	 * Calculate adjusted clock pixel and find out what the PLL can
+	 * produce. The PLL is limited, so the clock might be inaccurate.
+	 */
 	ret = tc_pxl_pll_calc(tc, clk_get_rate(tc->refclk),
 			      crtc_state->mode.clock * 1000,
-			      &adjusted_clock, NULL);
+			      &adjusted_clock_0p, NULL);
+	if (ret)
+		return ret;
+
+	/*
+	 * Calculate adjusted pixel clock with 1% faster requested pixel
+	 * clock and find out what the PLL can produce. This may in fact
+	 * be closer to the expected pixel clock of the output.
+	 */
+	ret = tc_pxl_pll_calc(tc, clk_get_rate(tc->refclk),
+			      crtc_state->mode.clock * 1010,
+			      &adjusted_clock_1p, NULL);
 	if (ret)
 		return ret;
 
+	/* Pick the more accurate of the two calculated results. */
+	if (crtc_state->mode.clock * 1010 - adjusted_clock_1p <
+	    crtc_state->mode.clock * 1000 - adjusted_clock_0p)
+		adjusted_clock = adjusted_clock_1p;
+	else
+		adjusted_clock = adjusted_clock_0p;
+
 	crtc_state->adjusted_mode.clock = adjusted_clock / 1000;
 
 	/* DSI->DPI interface clock limitation: upto 100 MHz */
-- 
2.45.2