linux-kernel - Re: [PATCH 08/12] drm/sun4i: sun6i_mipi_dsi: Refactor vertical video start delay

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Date:   Sat, 29 Sep 2018 17:27:58 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Jagan Teki <jagan@...rulasolutions.com>
Cc:     Chen-Yu Tsai <wens@...e.org>, Icenowy Zheng <icenowy@...c.io>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        Vasily Khoruzhick <anarsoul@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        David Airlie <airlied@...ux.ie>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        Michael Trimarchi <michael@...rulasolutions.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        devicetree <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [PATCH 08/12] drm/sun4i: sun6i_mipi_dsi: Refactor vertical video
 start delay

On Thu, Sep 27, 2018 at 11:03:19PM +0530, Jagan Teki wrote:
> On Thu, Sep 27, 2018 at 10:44 PM Maxime Ripard
> <maxime.ripard@...tlin.com> wrote:
> >
> > On Thu, Sep 27, 2018 at 05:18:46PM +0530, Jagan Teki wrote:
> > > Accordingly to BPI-M64-bsp DE DSI code Video start delay
> > > can be computed by subtracting total vertical timing with
> > > front porch timing and with adding 1 delay line for TCON.
> >
> > This is what the current code is doing as well.
> 
> The current code
> return mode->vtotal - (mode->vsync_end - mode->vdisplay) + 1;
> 
> (mode->vsync_end - mode->vdisplay) = front porch + sync
> 
> but I'm updating here only front porch.
> 
> >
> > > This patch simply add the start_delay logic from BPI-M64-bsp,
> > > w/o this new computation, the DSI on A64 encounter vblank time out.
> > >
> > > [CRTC:36:crtc-0] vblank wait timed out
> > >
> > > Signed-off-by: Jagan Teki <jagan@...rulasolutions.com>
> > > ---
> > >  drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 12 +++++++++++-
> > >  1 file changed, 11 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > index 9918fdb990ff..217db74c6dc3 100644
> > > --- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > +++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
> > > @@ -358,7 +358,17 @@ 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;
> > > +     u32 vfp = mode->vsync_start - mode->vdisplay;
> 
> let me explain this.
> 
> Actual code from Allwinner
> u32 vfp = panel->lcd_vt - panel->lcd_y - panel->lcd_vbp;
> 
> So,
> 
> => (panel->lcd_vt - panel->lcd_y) - (panel->lcd_vbp)
> => (front porch + sync + back porch) - (back porch + sync)

Unless Allwinner is doing something fishy, in which case that should
be mentionned, the back porch doesn't contain the sync pulse.

Maxime

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

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