This is a note to let you know that I've just added the patch titled
drm/vc4: dsi: Correct DSI divider calculations
to the 4.19-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
drm-vc4-dsi-correct-dsi-divider-calculations.patch
and it can be found in the queue-4.19 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit e542050bce9df7b09ef114ac7e355abb89c75281
Author: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>
Date: Mon Jun 13 16:47:39 2022 +0200
drm/vc4: dsi: Correct DSI divider calculations
[ Upstream commit 3b45eee87da171caa28f61240ddb5c21170cda53 ]
The divider calculations tried to find the divider just faster than the
clock requested. However if it required a divider of 7 then the for loop
aborted without handling the "error" case, and could end up with a clock
lower than requested.
The integer divider from parent PLL to DSI clock is also capable of
going up to /255, not just /7 that the driver was trying. This allows
for slower link frequencies on the DSI bus where the resolution permits.
Correct the loop so that we always have a clock greater than requested,
and covering the whole range of dividers.
Fixes: 86c1b9eff3f2 ("drm/vc4: Adjust modes in DSI to work around the integer PLL divider.")
Signed-off-by: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220613144800.326124-13-maxime@xxxxxxxxxx
Signed-off-by: Maxime Ripard <maxime@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
index 0c607eb33d7e..77003ce666a4 100644
--- a/drivers/gpu/drm/vc4/vc4_dsi.c
+++ b/drivers/gpu/drm/vc4/vc4_dsi.c
@@ -853,11 +853,9 @@ static bool vc4_dsi_encoder_mode_fixup(struct drm_encoder *encoder,
/* Find what divider gets us a faster clock than the requested
* pixel clock.
*/
- for (divider = 1; divider < 8; divider++) {
- if (parent_rate / divider < pll_clock) {
- divider--;
+ for (divider = 1; divider < 255; divider++) {
+ if (parent_rate / (divider + 1) < pll_clock)
break;
- }
}
/* Now that we've picked a PLL divider, calculate back to its
