Re: [PATCH 1/3] drm/i915: Use the the correct pixel rate to compute wm line time

["Govindapillai, Vinod" <vinod.govindapillai@intel.com>]

On Fri, 2025-09-19 at 21:08 +0300, Ville Syrjala wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> The line time used for the watermark calculations is supposed to
> based on the plane's adjusted pixel rate, not the pipe's adjusted
> pixel rate. The current code will give incorrect answers if plane
> downscaling is used.
> 
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/display/skl_watermark.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/skl_watermark.c
> b/drivers/gpu/drm/i915/display/skl_watermark.c
> index d74cbb43ae6f..bdd005c6cc2d 100644
> --- a/drivers/gpu/drm/i915/display/skl_watermark.c
> +++ b/drivers/gpu/drm/i915/display/skl_watermark.c
> @@ -1637,18 +1637,16 @@ skl_wm_method2(u32 pixel_rate, u32 pipe_htotal, u32 latency,
>  }
>  
>  static uint_fixed_16_16_t
> -intel_get_linetime_us(const struct intel_crtc_state *crtc_state)
> +intel_get_linetime_us(const struct intel_crtc_state *crtc_state,
> +		      int pixel_rate)
>  {
>  	struct intel_display *display = to_intel_display(crtc_state);
> -	u32 pixel_rate;
>  	u32 crtc_htotal;
>  	uint_fixed_16_16_t linetime_us;
>  
>  	if (!crtc_state->hw.active)
>  		return u32_to_fixed16(0);
>  
> -	pixel_rate = crtc_state->pixel_rate;
> -
>  	if (drm_WARN_ON(display->drm, pixel_rate == 0))
>  		return u32_to_fixed16(0);
>  
> @@ -1743,7 +1741,8 @@ skl_compute_wm_params(const struct intel_crtc_state *crtc_state,
>  	wp->y_tile_minimum = mul_u32_fixed16(wp->y_min_scanlines,
>  					     wp->plane_blocks_per_line);
>  
> -	wp->linetime_us = fixed16_to_u32_round_up(intel_get_linetime_us(crtc_state));
> +	wp->linetime_us = fixed16_to_u32_round_up(intel_get_linetime_us(crtc_state,
> +									plane_pixel_rate));

Hi Ville,

As per the bspec 49325 the starting point is,

Adjusted pipe pixel rate = pixel rate for the screen resolution
if (pipe scale enabled)
	adjusted pipe pixel rate = adjusted pipe pixel rate * pipe down scale amoun

adjusted plane pixel rate = adjusted pipe pixel rate
if (plane scale enabled)
	adjusted plane pixel rate = adjusted plane pixel rate * plane down scale 

and
line time microseconds = pipe horizontal total pixels/adjusted plane pixel rate MHz

Our Method1, Method2 and line time calculations are based on plane_pixel_rate vs. adjusted plane
pixel rate in bspec. So I wonder if we differ from the bspec in these wm calculations?

BR
Vinod
	


>  
>  	return 0;
>  }