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

[Maxime Ripard]

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.

> 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@xxxxxxxxxxxxxxxxxxxx>
> ---
>  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;

Again, this is wrong and contrary to what your commit log is saying.

The Allwinner code has:

u32 vfp = panel->lcd_vt - panel->lcd_y - panel->lcd_vbp;
u32 dsi_start_delay = panel->lcd_vt - vfp + 1;

So, essentially:

vtotal - (vtotal - vdisplay - back porch) + 1

The backporch is hsync_total - hsync_end, so we end up, removing the
addition / removal of vtotal, with:

vdisplay - (vsync_total - vsync_end) + 1

The formula used there looks indeed different, but unlike what you
were saying.

> +	u32 start_delay;
> +
> +	start_delay = mode->vtotal - vfp + 1;
> +	if (start_delay > mode->vtotal)
> +		start_delay -= mode->vtotal;
> +
> +	if (!start_delay)
> +		start_delay = 1;
> +

I guess that it's actually the clamping that fixes thing. It should be
mentionned in your commit log.

Maxime

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