Re: [PATCH v3 2/8] drm/sun4i: dsi: Change the start delay calculation

From: Maxime Ripard <maxime.ripard@bootlin.com>
To: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Cc: bbrezillon@kernel.org,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	dri-devel@lists.freedesktop.org, Chen-Yu Tsai <wens@csie.org>,
	Sean Paul <seanpaul@chromium.org>,
	Thomas Petazzoni <thomas.petazzoni@bootlin.com>,
	Jagan Teki <jagan@openedev.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 2/8] drm/sun4i: dsi: Change the start delay calculation
Date: Tue, 12 Feb 2019 16:45:32 +0100	[thread overview]
Message-ID: <20190212154532.uwntofq7hkm74xjv@flea> (raw)
In-Reply-To: <0f3247c920d1f5ce67cc7e024125c3a4d3ca0c81.camel@bootlin.com>

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On Tue, Feb 12, 2019 at 04:28:13PM +0100, Paul Kocialkowski wrote:
> Hi,
> 
> On Mon, 2019-02-11 at 15:41 +0100, Maxime Ripard wrote:
> > The current calculation for the video start delay in the current DSI driver
> > is that it is the total vertical size, minus the front porch and sync length,
> > plus 1. This equals to the active vertical size plus the back porch plus 1.
> > 
> > That 1 is coming in the Allwinner BSP from an variable that is set to 1.
> > However, if we look at the Allwinner BSP more closely, and especially in
> > the "legacy" code for the display (in drivers/video/sunxi/legacy/), we can
> > see that this variable is actually computed from the porches and the sync
> > minus 10, clamped between 8 and 100.
> 
> For future reference, it look like there is a version of the BSP where
> this is clamped between 8 and 31 in the legacy ebios code:
> https://github.com/Icenowy/linux-kernel-lichee-a33/blob/master/drivers/video/sunxi/legacy/disp/de_bsp/de/ebios/de_dsi.c#L687
> 
> As well as in the A23 code:
> https://github.com/Icenowy/linux-kernel-lichee-a33/blob/master/drivers/video/sunxi/disp/de/lowlevel_sun8iw3/de_dsi.c#L686
> 
> > This fixes the start delay symptom we've seen on some panels (vblank
> > timeouts with vertical white stripes at the bottom of the panel).
> > 
> > Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
> > ---
> >  drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > index 380fc527a707..9471fa695ec7 100644
> > --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > @@ -357,7 +357,9 @@ 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)
> >  {
> > -	return mode->vtotal - (mode->vsync_end - mode->vdisplay) + 1;
> > +	u16 start = clamp(mode->vtotal - mode->vdisplay - 10, 8, 100);
> > +
> > +	return mode->vtotal - (mode->vsync_end - mode->vdisplay) + start;
> 
> Looking at the formula from the BSP, I get a different result from
> yours.
> 
> From what I can see, the BSP formula is:
> start_delay = vtotal - vfp + start
> vfp = vtotal - y - vbp
> 
> With vbp counting the back porch and sync length.

vbp is only the back porch.

Maxime

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

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