Re: [PATCH v3 3/8] drm/sun4i: dsi: Enforce boundaries on the start delay

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 3/8] drm/sun4i: dsi: Enforce boundaries on the start delay
Date: Thu, 14 Feb 2019 10:21:48 +0100	[thread overview]
Message-ID: <20190214092148.3jhtp5i5crl632wd@flea> (raw)
In-Reply-To: <bf6176bef13d471a1fb829f918dbbe38de909e21.camel@bootlin.com>

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hi,

On Tue, Feb 12, 2019 at 04:30:33PM +0100, Paul Kocialkowski wrote:
> Hi,
> 
> On Mon, 2019-02-11 at 15:41 +0100, Maxime Ripard wrote:
> > The Allwinner BSP makes sure that we don't end up with a null start delay
> > or with a delay larger than vtotal.
> > 
> > The former condition is likely to happen now with the reworked start delay,
> > so make sure we enforce the same boundaries.
> > 
> > Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
> 
> Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>

Thanks!

> > ---
> >  drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > index 9471fa695ec7..506f2e8cf454 100644
> > --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > @@ -358,8 +358,12 @@ 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;
> >  
> > -	return mode->vtotal - (mode->vsync_end - mode->vdisplay) + start;
> > +	if (delay > mode->vtotal)
> > +		delay = delay % mode->vtotal;
> 
> I wonder whether delay == mode->vtotal would make sense at all. If not,
> then we can just apply the modulo unconditionally. But in doubt, let's
> keep things this way.

I guess that situation can happen, especially with the start clamping.

But even without it, it can happen when mode->vtotal - mode->vsync_end
(so the backporch) is equal to 10.

Maxime

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

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