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

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Date:   Thu, 27 Sep 2018 23:03:19 +0530
From:   Jagan Teki <jagan@...rulasolutions.com>
To:     Maxime Ripard <maxime.ripard@...tlin.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 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)
=> front porch + sync + back porch - back porch - sync
=> front porch
=> mode->vsync_start - mode->vdisplay

Hope it clear.