[Intel-gfx] [PATCH] drm/i915: Fail if DSC compression requirement is less than platform supports

[Luca Coelho]

On Mon, 2023-06-26 at 11:28 +0300, Stanislav Lisovskiy wrote:
> Currently we just clamp that value to the highest supported one, however that
> means, we are not able to fit this into our available bandwidth range, so we
> might see glitches or FIFO underruns.
> While choosing less compressed bpp than min bpp required to handle the mode is
> harmless and might even save some bandwidth, choosing higher compressed bpp than
> min bpp required to handle the required mode config, can cause issues.
> So in that case lets just conclude that even with DSC, we are not able to comply
> with bandwidth requirements and fail.
> 
> Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 9f40da20e88d..8696a1f02805 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -713,8 +713,17 @@ u32 intel_dp_dsc_nearest_valid_bpp(struct drm_i915_private *i915, u32 bpp, u32 p
>  
>  		/*
>  		 * According to BSpec, 27 is the max DSC output bpp,
> -		 * 8 is the min DSC output bpp
> +		 * 8 is the min DSC output bpp.
> +		 * While we can still clamp higher bpp values to 27, saving bandwidth,
> +		 * if it is required to oompress up to bpp < 8, means we can't do

Small typo, "compress".


> +		 * that and probably means we can't fit the required mode, even with
> +		 * DSC enabled.
>  		 */
> +		if (bits_per_pixel < 8) {
> +			drm_dbg_kms(&i915->drm, "Unsupported BPP %u, min 8\n",
> +				    bits_per_pixel);
> +			return 0;
> +		}
>  		bits_per_pixel = clamp_t(u32, bits_per_pixel, 8, 27);

I guess you don't need to clamp anymore but could use min_t(u32,
bits_per_pixel, 27) now, right? Actually, you don't even need to type
it, so min(bits_per_pixel, 27) should suffice.


>  	} else {
>  		/* Find the nearest match in the array of known BPPs from VESA */

--
Cheers,
Luca.