On Thu, Mar 07, 2019 at 11:24:01PM +0530, Jagan Teki wrote:
> Hi Maxime,
> 
> On Fri, Feb 8, 2019 at 3:23 PM Maxime Ripard <maxime.ripard@bootlin.com> 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.
> >
> > 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;
> >  }
> 
> As you stated in commit message about result as "active vertical size
> + Back porch" which indeed correct in reverse way of previous code.
> but the change seems incorrect to me.
>
> => mode->vtotal - (mode->vsync_end - mode->vdisplay) + start
> => mode->vtotal - (mode->vsync_end - mode->vdisplay) + (mode->vtotal -
> mode->vdisplay) # bypass 10 for now
> => mode->vtotal - mode->vsync_end +  mode->vdisplay + mode->vtotal -
> mode->vdisplay
> => (mode->vtotal - mode->vsync_end) + mode->vtotal
> => Back porch + mode->vtotal
> 
> but not
> => Back porch + mode->vdisplay

Why do you think it should be this result?

Maxime

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