drm/vc4: dsi: Correct DSI divider calculations

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@raspberrypi.com>
Link: https://lore.kernel.org/r/20220613144800.326124-13-maxime@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
index e82ee94..81a6c4e 100644 (file)
--- 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