[PATCH v3] drm/i915/display: Use the recomended min_hblank values

[Imre Deak]

On Wed, Jun 25, 2025 at 11:04:07AM +0530, Arun R Murthy wrote:
> Use recommended values as per wa_14021694213 to compare with the
> calculated value and choose minimum of them.
> 
> v2: corrected checkpatch warning and retain the restriction for
> min_hblank (Jani)
> v3: use calculated value to compare with recomended value and choose
> minimum of them (Imre)
> 
> Bspec: 74379
> Signed-off-by: Arun R Murthy <arun.r.murthy at intel.com>
> ---
> Changes in v3:
> - EDITME: describe what is new in this series revision.
> - EDITME: use bulletpoints and terse descriptions.
> - Link to v2: https://lore.kernel.org/r/20250624-min_hblank-v2-1-9110a9342d8c@intel.com
> 
> Changes in v2:
> - EDITME: describe what is new in this series revision.
> - EDITME: use bulletpoints and terse descriptions.
> - Link to v1: https://lore.kernel.org/r/20250624-min_hblank-v1-1-5c100e4a8b05@intel.com

The above 'Changes in vx' templates could be removed imo.

> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index f48912f308df7fd26c9d089e8f1bd096bfc8df95..c55315fb74734fded40695bae40dd19f71a9786f 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -3175,7 +3175,22 @@ int intel_dp_compute_min_hblank(struct intel_crtc_state *crtc_state,
>  	 */
>  	min_hblank = min_hblank - 2;
>  
> -	min_hblank = min(10, min_hblank);
> +	/*
> +	 * min_hblank formula is undergoing a change, to avoid underrun use the
> +	 * recomended value in spec to compare with the calculated one and use the
> +	 * minimum value
> +	 */
> +	if (intel_dp_is_uhbr(crtc_state)) {
> +		if (crtc_state->dsc.compression_enable &&
> +		    crtc_state->output_format == INTEL_OUTPUT_FORMAT_YCBCR420 &&
> +		    crtc_state->pipe_bpp == 18)

The bspec page describes this format as YUV420 at 6 bpp. Not sure how
that would align with pipe_bpp == 18 (which is 6 _bpc_). Bspec could
also refer to the compressed bpp, however the driver's minimum
compressed bpp is 8. Could you clarify which bpp bspec refers to?

> +			min_hblank = min(2, min_hblank);
> +		else
> +			min_hblank = min(3, min_hblank);
> +	} else {
> +		min_hblank = min(10, min_hblank);
> +	}
> +
>  	crtc_state->min_hblank = min_hblank;
>  
>  	return 0;
> 
> ---
> base-commit: da56936fad6f5e1d5f0cef8b50277bfb071eefe7
> change-id: 20250624-min_hblank-8af8d2626ff8
> 
> Best regards,
> -- 
> Arun R Murthy <arun.r.murthy at intel.com>
>