Re: [PATCH 1/2] drm/i915/dsc: Fix QP table index calculation for bpp selection

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] drm/i915/dsc: incomplete fix due to unscaled fractional_bits in QP index calculation
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commit f16f36523ce6f8e8fca7d8b3376a00ba0a384a25
Author: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Subject: drm/i915/dsc: Fix QP table index calculation for bpp selection

This patch fixes the QP table index calculation for a given bpp by restoring
a missing multiplier of 2 for the integer portion of the calculation. This
aims to ensure proper table layout alignment.

> diff --git a/drivers/gpu/drm/i915/display/intel_vdsc.c b/drivers/gpu/drm/i915/display/intel_vdsc.c
> index 8f06c3a4d56df..7a573c2b6a7dd 100644
> --- a/drivers/gpu/drm/i915/display/intel_vdsc.c
> +++ b/drivers/gpu/drm/i915/display/intel_vdsc.c
[ ... ]
> @@ -261,7 +261,7 @@ calculate_rc_params(struct drm_dsc_config *vdsc_cfg)
>  		 * and so on.
>  		 * 0.5 fractional part with 4 decimal precision becomes 5000
>  		 */
> -		bpp_i  = ((bpp - 6) + (fractional_bits < 5000 ? 0 : 1));
> +		bpp_i  = (2 * (bpp - 6) + (fractional_bits < 5000 ? 0 : 1));

[Severity: Medium]
Does this calculation completely fix the index calculation, or is there a
missing scaling factor for fractional_bits?

Earlier in calculate_rc_params(), fractional_bits is extracted directly from
a Q4 value:
    
    int fractional_bits = fxp_q4_to_frac(vdsc_cfg->bits_per_pixel);

Since fxp_q4_to_frac() returns the raw Q4 fractional bits (which are strictly
in the range 0-15), the condition (fractional_bits < 5000) will always evaluate
to true. 

Because of this, the expression (fractional_bits < 5000 ? 0 : 1) will always
evaluate to 0, which means the 0.5 fractional bpp increment is ignored. Is
fractional_bits missing the 10000 scaling factor expected by this logic?

>  
>  		for (buf_i = 0; buf_i < DSC_NUM_BUF_RANGES; buf_i++) {
>  			u8 range_bpg_offset;

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Sashiko AI review · https://sashiko.dev/#/patchset/20261016101713.230013-1-ankit.k.nautiyal@intel.com?part=1