On Fri, 10 Jan 2020, Stanislav Lisovskiy <stanislav.lisovs...@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.
> We've found experimentally that raising cdclk to
> at least pixel_rate (rather than pixel_rate/2)
> eliminates these underruns, so let's use this as a
> temporary workaround until the hardware team
> can suggest a more precise remedy.
>
> Explicitly stating here that this seems to be currently
> rather a Hack, than final solution.
>
> v2: Use clamp operation instead of min(Matt Roper)
>
> v3: - Fixed commit message(Matt Roper)
> - Now using pixel_rate instead of max_cdclk(Jani Nikula)
> - Switched to max from clamp(Ville Syrjälä)
> Hopefully this hybrid satisfies everyone :)
>
> Reviewed-by: Matt Roper <matthew.d.ro...@intel.com>
> Signed-off-by: Stanislav Lisovskiy <stanislav.lisovs...@intel.com>
> Closes: https://gitlab.freedesktop.org/drm/intel/issues/402
Pushed to dinq, thanks for the patch and review.
And now to figure out the permanent fix!
BR,
Jani.
> ---
> drivers/gpu/drm/i915/display/intel_cdclk.c | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c
> b/drivers/gpu/drm/i915/display/intel_cdclk.c
> index 7d1ab1e5b7c3..0ce5926006ca 100644
> --- a/drivers/gpu/drm/i915/display/intel_cdclk.c
> +++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
> @@ -2004,6 +2004,18 @@ 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 = max(min_cdclk, (int)crtc_state->pixel_rate);
> +
> 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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