Re: [Intel-gfx] [PATCH v2] drm/i915: Bump up CDCLK to eliminate underruns on TGL

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 09 Jan 2020, Stanislav Lisovskiy <stanislav.lisovskiy@intel.com> wrote:
> There seems to be some undocumented bandwidth
> bottleneck/dependency which scales with CDCLK,
> causing FIFO underruns when CDCLK is too low,
> even when it's correct from BSpec point of view.
>
> Currently for TGL platforms we calculate
> min_cdclk initially based on pixel_rate divided
> by 2, accounting for also plane requirements,
> however in some cases the lowest possible CDCLK
> doesn't work and causing the underruns.
>
> Explicitly stating here that this seems to be currently
> rather a Hack, than final solution.
>
> v2: Use clamp operation instead of min(Matt Roper)
>
> Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
> Bugzilla: https://gitlab.freedesktop.org/drm/intel/issues/402
> ---
>  drivers/gpu/drm/i915/display/intel_cdclk.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
> index 7d1ab1e5b7c3..23ef30175090 100644
> --- a/drivers/gpu/drm/i915/display/intel_cdclk.c
> +++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
> @@ -2004,6 +2004,20 @@ int intel_crtc_compute_min_cdclk(const struct intel_crtc_state *crtc_state)
>  	/* Account for additional needs from the planes */
>  	min_cdclk = max(intel_planes_min_cdclk(crtc_state), min_cdclk);
>  
> +	/*
> +	 * HACK. Currently for TGL platforms we calculate
> +	 * min_cdclk initially based on pixel_rate divided
> +	 * by 2, accounting for also plane requirements,
> +	 * however in some cases the lowest possible CDCLK
> +	 * doesn't work and causing the underruns.
> +	 * Explicitly stating here that this seems to be currently
> +	 * rather a Hack, than final solution.
> +	 */
> +	if (IS_TIGERLAKE(dev_priv))
> +		min_cdclk = clamp(min_cdclk,
> +				  (int)crtc_state->pixel_rate,
> +				  (int)dev_priv->max_cdclk_freq);

I don't think you can do that even as a hack. By clamping the required
minimum to the max cdclk, you lose the check below. Can't do that.

BR,
Jani.

> +
>  	if (min_cdclk > dev_priv->max_cdclk_freq) {
>  		DRM_DEBUG_KMS("required cdclk (%d kHz) exceeds max (%d kHz)\n",
>  			      min_cdclk, dev_priv->max_cdclk_freq);

-- 
Jani Nikula, Intel Open Source Graphics Center
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