[PATCH v2 3/9] drm/sun4i: dsi: Change the start delay calculation

[Jagan Teki]

Hi Maxime,

On Fri, Feb 8, 2019 at 3:23 PM Maxime Ripard <maxime.ripard at 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 at 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

Let me know if I'm wrong?