linux-kernel - Re: [PATCH v8 01/15] drm/sun4i: dsi: Fix video start delay computation

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Date:   Mon, 11 Mar 2019 16:37:34 +0100
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Chen-Yu Tsai <wens@...e.org>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-clk@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        devicetree@...r.kernel.org,
        Michael Trimarchi <michael@...rulasolutions.com>,
        linux-amarula@...rulasolutions.com, linux-sunxi@...glegroups.com
Subject: Re: [PATCH v8 01/15] drm/sun4i: dsi: Fix video start delay
 computation

On Mon, Mar 11, 2019 at 07:06:23PM +0530, Jagan Teki wrote:
> Vertical video start delay is computed by excluding vertical front
> porch value from total vertical timings.
> 
> This clearly confirmed from BSP code and here how it computed,
> 
> (drivers/video/sunxi/disp2/disp/de/lowlevel_sun50iw1/de_dsi.c)
> u32 vfp = panel->lcd_vt - panel->lcd_y - panel->lcd_vbp;
> => (panel->lcd_vt) - panel->lcd_y - (panel->lcd_vbp)
> => (timmings->ver_front_porch + panel->lcd_vbp + panel->lcd_y)
>    - panel->lcd_y - (panel->lcd_vbp)
> => timmings->ver_front_porch + panel->lcd_vbp + panel->lcd_y
>   			     - panel->lcd_y - panel->lcd_vbp
> => timmings->ver_front_porch
> 
> But the current driver is assuming it can exclude vertical front
> porch along with vertical sync values from total vertical timings,
> which resulting wrong start delay indeed wrong picture rendering
> in the panel.

Same story here: which panel, which datasheet, which "wrong picture
rendering"?

> Example: timings, where it produces the issue.
> {
> 	.vdisplay	= 600,
> 	.vsync_start	= 600 + 12,
> 	.vsync_end	= 600 + 12 + 2,
> 	.vtotal		= 600 + 12 + 2 + 21,
> }

Can you 100% trust those timings?

> It produces the desired start delay value as 19 but the correct working
> value should be 513.
>
> So, Fix it by computing proper video start delay.
> 
> Fixes: 69006ef0ecb1 ("drm/sun4i: dsi: Change the start delay calculation")
> Signed-off-by: Jagan Teki <jagan@...rulasolutions.com>
> ---
>  drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> index 62a508420227..8d6292c0158b 100644
> --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> @@ -364,8 +364,14 @@ static void sun6i_dsi_inst_init(struct sun6i_dsi *dsi,
>  static u16 sun6i_dsi_get_video_start_delay(struct sun6i_dsi *dsi,
>  					   struct drm_display_mode *mode)
>  {
> -	u16 start = clamp(mode->vtotal - mode->vdisplay - 10, 8, 100);
> -	u16 delay = mode->vtotal - (mode->vsync_end - mode->vdisplay) + start;
> +	u16 delay = mode->vtotal - (mode->vsync_start - mode->vdisplay);
> +
> +	/**
> +	 * BSP comment:
> +	 * put start_delay to tcon. set ready sync early to dramfreq,
> +	 * so set start_delay 1
> +	 */

That doesn't make any sense to me... What does it mean?

Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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