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

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Date:   Thu, 18 Oct 2018 11:55:28 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Icenowy Zheng <icenowy@...c.io>
Cc:     David Airlie <airlied@...ux.ie>, Rob Herring <robh+dt@...nel.org>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-sunxi@...glegroups.com
Subject: Re: [PATCH 9/9] [DO NOT MERGE] drm/sun4i: rgb: Add 5% tolerance to dot clock frequency check

Hi Icenowy,

Thank you for the patch.

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