Re: [PATCH 9/9] [DO NOT MERGE] drm/sun4i: rgb: Add 5% tolerance to dot clock frequency check

Laurent Pinchart
2018-10-18T08:55:28Z

Hi Icenowy,

Thank you for the patch.

On Thursday, 18 October 2018 10:33:27 EEST Icenowy Zheng wrote:
> From: Chen-Yu Tsai <wens-jdAy2FN1RRM@public.gmane.org>
>
> 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.

Why do you think this shouldn't be merged ?

> Signed-off-by: Chen-Yu Tsai <wens-jdAy2FN1RRM@public.gmane.org>
> ---
> 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 bf068da6b12e..23bdc449eacc 100644
> --- a/drivers/gpu/drm/sun4i/sun4i_rgb.c
> +++ b/drivers/gpu/drm/sun4i/sun4i_rgb.c
> @@ -92,13 +92,14 @@ static enum drm_mode_status sun4i_rgb_mode_valid(struct
> drm_encoder *crtc,
>
> DRM_DEBUG_DRIVER("Vertical parameters OK\n");
>
> + /* Check against a 5% tolerance for the dot clock */
> tcon->dclk_min_div = 6;
> tcon->dclk_max_div = 127;
> 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");

--
Regards,

Laurent Pinchart