[PATCH 05/20] drm/bridge: tc358775: add crtc modes fixup

[Michael Walle]

The bridge has some limitations regarding the horizontal display
timings. In particular, the pulse width has to be at least 8 pixels
and all horizontal timings have to be a multiple of two pixels, except
for the front porch which is ignored by the bridge anyway.

To accommodate that, add pixels to the pulse width and the back porch
until these requirements are satisfied. The added pixels are then
substracted from the front porch so we don't actually change the pixel
clock (or framerate).

Fixes: b26975593b17 ("display/drm/bridge: TC358775 DSI/LVDS driver")
Signed-off-by: Michael Walle <mwalle at kernel.org>
---
 drivers/gpu/drm/bridge/tc358775.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/drivers/gpu/drm/bridge/tc358775.c b/drivers/gpu/drm/bridge/tc358775.c
index 980f71ea5a6a..720c0d63fd6a 100644
--- a/drivers/gpu/drm/bridge/tc358775.c
+++ b/drivers/gpu/drm/bridge/tc358775.c
@@ -502,6 +502,37 @@ static void tc_bridge_enable(struct drm_bridge *bridge)
 	d2l_write(tc->i2c, LVCFG, val);
 }
 
+/*
+ * According to the datasheet, the horizontal back porch, front porch and sync
+ * length must be a multiple of 2 and the minimal horizontal pulse width is 8.
+ * To workaround this, we modify the back porch and the sync pulse width by
+ * adding enough pixels. These pixels will then be substracted from the front
+ * porch which is ignored by the bridge.  Hopefully, this marginal modified
+ * timing is tolerated by the panel. The alternative is either a black screen
+ * (if the sync pulse width is too short or a shifted picture if the lengths
+ * are not even).
+ */
+static bool tc_mode_fixup(struct drm_bridge *bridge,
+			  const struct drm_display_mode *mode,
+			  struct drm_display_mode *adj)
+{
+	u16 hsync_len, hback_porch;
+
+	hback_porch = adj->htotal - adj->hsync_end;
+	if (hback_porch & 1) {
+		adj->hsync_end -= 1;
+		adj->hsync_start -= 1;
+	}
+
+	hsync_len = adj->hsync_end - adj->hsync_start;
+	if (hsync_len < 8)
+		adj->hsync_start -= 8 - hsync_len;
+	else if (hsync_len & 1)
+		adj->hsync_start -= 1;
+
+	return adj->hsync_start >= adj->hdisplay;
+}
+
 static enum drm_mode_status
 tc_mode_valid(struct drm_bridge *bridge,
 	      const struct drm_display_info *info,
@@ -603,6 +634,7 @@ static const struct drm_bridge_funcs tc_bridge_funcs = {
 	.attach = tc_bridge_attach,
 	.pre_enable = tc_bridge_pre_enable,
 	.enable = tc_bridge_enable,
+	.mode_fixup = tc_mode_fixup,
 	.mode_valid = tc_mode_valid,
 	.post_disable = tc_bridge_post_disable,
 };

-- 
2.39.2