[12/33] drm/vc4: dsi: Correct DSI divider calculations

Commit Message

Maxime Ripard June 13, 2022, 2:47 p.m. UTC

From: Dave Stevenson <[e-mail]>

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 <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>
---
 drivers/gpu/drm/vc4/vc4_dsi.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

Patch

diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
index e82ee94cafc7..81a6c4e9576d 100644
--- a/drivers/gpu/drm/vc4/vc4_dsi.c
+++ b/drivers/gpu/drm/vc4/vc4_dsi.c
@@ -805,11 +805,9 @@  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