[PATCH v2025.09.y 45/58] video: mode-helpers: preserve sync polarity in fb_videomode conversion

[Ahmad Fatoum]

From: Ahmad Fatoum <a.fatoum at barebox.org>

The FB_SYNC_*_HIGH_ACT flags indicate that the respective sync signal is
active high when set and active low when unset.

The DISPLAY_FLAGS_*SYNC_* flags on the other hands have separate
non-zero _HIGH and _LOW flags, which fb_videomode_to_videomode() failed
to account for.

This causes a DPI sync polarity mismatch for MIPI-DSI panels with
negative sync polarity, which was hackily worked around by enabling the
video pattern generator intermittently.

Fix by explicitly setting the LOW polarity flags, which restores
DRM_MODE_FLAG_NHSYNC/NVSYNC after the fb_videomode round-trip.

(cherry picked from commit f66af1ff3a2042bfbadbfc952359d1cdc6fb07c3)

Reported-by: Claude Opus 4.6 <noreply at anthropic.com>
Fixes: 7fc6f7b5daa0 ("video: add videomode helpers")
Signed-off-by: Ahmad Fatoum <a.fatoum at barebox.org>
Link: https://lore.barebox.org/20260213092906.3960051-1-a.fatoum@barebox.org
Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>
---
 drivers/video/mode-helpers.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/video/mode-helpers.c b/drivers/video/mode-helpers.c
index 4a1d31813e79..89abd8342bf4 100644
--- a/drivers/video/mode-helpers.c
+++ b/drivers/video/mode-helpers.c
@@ -143,8 +143,12 @@ void fb_videomode_to_videomode(const struct fb_videomode *fbmode,
 
 	if (fbmode->sync & FB_SYNC_HOR_HIGH_ACT)
 		vm->flags |= DISPLAY_FLAGS_HSYNC_HIGH;
+	else
+		vm->flags |= DISPLAY_FLAGS_HSYNC_LOW;
 	if (fbmode->sync & FB_SYNC_VERT_HIGH_ACT)
 		vm->flags |= DISPLAY_FLAGS_VSYNC_HIGH;
+	else
+		vm->flags |= DISPLAY_FLAGS_VSYNC_LOW;
 	if (fbmode->vmode & FB_VMODE_INTERLACED)
 		vm->flags |= DISPLAY_FLAGS_INTERLACED;
 	if (fbmode->vmode & FB_VMODE_DOUBLE)
-- 
2.47.3