[PATCH v2 2/2] 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
from either mode clock or display_timing .pixelclock.max , whichever is
higher. Since the Pixel PLL output clock frequency calculation always
returns lower frequency than the requested clock frequency, passing in
the higher clock frequency should result in output clock frequency which
is closer to the expected pixel clock.

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.

Keep the change isolated to DPI output.

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
---
V2: - Isolate the change to DPI only, split tc_bridge_mode_set()
    - Look up display_timings and use .pixelclock.max as input
      into the PLL calculation if available. That should yield
      more accurate results for DPI panels.
---
 drivers/gpu/drm/bridge/tc358767.c | 47 +++++++++++++++++++++++++------
 1 file changed, 39 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/bridge/tc358767.c b/drivers/gpu/drm/bridge/tc358767.c
index 0d523322fdd8e..fe9ab06d82d91 100644
--- a/drivers/gpu/drm/bridge/tc358767.c
+++ b/drivers/gpu/drm/bridge/tc358767.c
@@ -39,6 +39,8 @@
 #include <drm/drm_print.h>
 #include <drm/drm_probe_helper.h>
 
+#include <video/display_timing.h>
+
 /* Registers */
 
 /* DSI D-PHY Layer registers */
@@ -1681,13 +1683,33 @@ static int tc_dpi_atomic_check(struct drm_bridge *bridge,
 			       struct drm_crtc_state *crtc_state,
 			       struct drm_connector_state *conn_state)
 {
+	u32 mode_clock = crtc_state->mode.clock * 1000;
 	struct tc_data *tc = bridge_to_tc(bridge);
-	int adjusted_clock = 0;
+	struct drm_bridge *nb = bridge;
+	struct display_timing timings;
+	struct drm_panel *panel;
+	int adjusted_clock;
 	int ret;
 
+	do {
+		if (!drm_bridge_is_panel(nb))
+			continue;
+
+		panel = drm_bridge_get_panel(nb);
+		if (!panel || !panel->funcs || !panel->funcs->get_timings)
+			continue;
+
+		ret = panel->funcs->get_timings(panel, 1, &timings);
+		if (ret <= 0)
+			break;
+
+		if (timings.pixelclock.max > mode_clock)
+			mode_clock = timings.pixelclock.max;
+		break;
+	} while ((nb = drm_bridge_get_next_bridge(nb)));
+
 	ret = tc_pxl_pll_calc(tc, clk_get_rate(tc->refclk),
-			      crtc_state->mode.clock * 1000,
-			      &adjusted_clock, NULL);
+			      mode_clock, &adjusted_clock, NULL);
 	if (ret)
 		return ret;
 
@@ -1758,9 +1780,18 @@ tc_edp_mode_valid(struct drm_bridge *bridge,
 	return MODE_OK;
 }
 
-static void tc_bridge_mode_set(struct drm_bridge *bridge,
-			       const struct drm_display_mode *mode,
-			       const struct drm_display_mode *adj)
+static void tc_dpi_bridge_mode_set(struct drm_bridge *bridge,
+				   const struct drm_display_mode *mode,
+				   const struct drm_display_mode *adj)
+{
+	struct tc_data *tc = bridge_to_tc(bridge);
+
+	drm_mode_copy(&tc->mode, adj);
+}
+
+static void tc_edp_bridge_mode_set(struct drm_bridge *bridge,
+				   const struct drm_display_mode *mode,
+				   const struct drm_display_mode *adj)
 {
 	struct tc_data *tc = bridge_to_tc(bridge);
 
@@ -1977,7 +2008,7 @@ tc_edp_atomic_get_output_bus_fmts(struct drm_bridge *bridge,
 static const struct drm_bridge_funcs tc_dpi_bridge_funcs = {
 	.attach = tc_dpi_bridge_attach,
 	.mode_valid = tc_dpi_mode_valid,
-	.mode_set = tc_bridge_mode_set,
+	.mode_set = tc_dpi_bridge_mode_set,
 	.atomic_check = tc_dpi_atomic_check,
 	.atomic_enable = tc_dpi_bridge_atomic_enable,
 	.atomic_disable = tc_dpi_bridge_atomic_disable,
@@ -1991,7 +2022,7 @@ static const struct drm_bridge_funcs tc_edp_bridge_funcs = {
 	.attach = tc_edp_bridge_attach,
 	.detach = tc_edp_bridge_detach,
 	.mode_valid = tc_edp_mode_valid,
-	.mode_set = tc_bridge_mode_set,
+	.mode_set = tc_edp_bridge_mode_set,
 	.atomic_check = tc_edp_atomic_check,
 	.atomic_enable = tc_edp_bridge_atomic_enable,
 	.atomic_disable = tc_edp_bridge_atomic_disable,
-- 
2.45.2