[PATCH RFC] drm/sun4i: rgb: Add 5% tolerance to dot clock frequency check

[Chen-Yu Tsai]

The panels shipped with Allwinner devices are very "generic", i.e.
they do not have model numbers or reliable sources of information
for the timings (that we know of) other than the fex files shipped
on them. The dot clock frequency provided in the fex files have all
been rounded to the nearest MHz, as that is the unit used in them.

We were using the simple panel "urt,umsh-8596md-t" as a substitute
for the A13 Q8 tablets in the absence of a specific model for what
may be many different but otherwise timing compatible panels. This
was usable without any visual artifacts or side effects, until the
dot clock rate check was added in commit bb43d40d7c83 ("drm/sun4i:
rgb: Validate the clock rate").

The reason this check fails is because the dotclock frequency for
this model is 33.26 MHz, which is not achievable with our dot clock
hardware, and the rate returned by clk_round_rate deviates slightly,
causing the driver to reject the display mode.

The LCD panels have some tolerance on the dot clock frequency, even
if it's not specified in their datasheets.

This patch adds a 5% tolerence to the dot clock check.

Signed-off-by: Chen-Yu Tsai <wens at csie.org>
---

The few LCD panel datasheets I found did not list minimums or maximums
for the dot clock rate. The 5% tolerance is just something I made up.
The point is to be able to use our dot clock, which doesn't have the
resolution needed to generate the exact clock rate requested. AFAIK
the sun4i driver is one of the strictest ones with regards to the dot
clock frequency. Some drivers don't even check it.

The clock rate given in vendor fex files are already rounded down to
MHz resolution. I doubt not using the exact rate as specified in simple
panels would cause any issues. But my experience is limited here.
Feedback on this is requested.

---
 drivers/gpu/drm/sun4i/sun4i_rgb.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/sun4i/sun4i_rgb.c b/drivers/gpu/drm/sun4i/sun4i_rgb.c
index d198ad7e5323..66ad86afa561 100644
--- a/drivers/gpu/drm/sun4i/sun4i_rgb.c
+++ b/drivers/gpu/drm/sun4i/sun4i_rgb.c
@@ -93,11 +93,12 @@ static int sun4i_rgb_mode_valid(struct drm_connector *connector,
 
 	DRM_DEBUG_DRIVER("Vertical parameters OK\n");
 
+	/* Check against a 5% tolerance for the dot clock */
 	rounded_rate = clk_round_rate(tcon->dclk, rate);
-	if (rounded_rate < rate)
+	if (rounded_rate < rate * 19 / 20)
 		return MODE_CLOCK_LOW;
 
-	if (rounded_rate > rate)
+	if (rounded_rate > rate * 21 / 20)
 		return MODE_CLOCK_HIGH;
 
 	DRM_DEBUG_DRIVER("Clock rate OK\n");
-- 
2.10.2