Re: [PATCH v3 04/10] drm/i915/dp: Add M/N ratio check with warning for DP link config

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] drm: log spam and DoS vector due to invariant drm_WARN inside bpp_x16 loop
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commit cb9847e0babf613d9ed13f234a331025ee93ec9c
Author: Ankit Nautiyal <ankit.k.nautiyal@intel.com>

drm/i915/dp: Add M/N ratio check with warning for DP link config

This commit introduces intel_dp_check_m_n_ratio() to validate the computed
link M/N ratio against the maximum allowed value. It issues a warning via
drm_WARN() if the limit is exceeded.

[ ... ]
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index 430ba1230c5ae..364f62563f904 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -347,6 +347,8 @@ int intel_dp_mtp_tu_compute_config(struct intel_dp *intel_dp,
>  					 link_bpp_x16,
>  					 &crtc_state->dp_m_n);
>  
> +		intel_dp_check_m_n_ratio(crtc_state, &crtc_state->dp_m_n);
		^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Severity: Medium]
Will this evaluate a loop-invariant condition on every iteration of the
bpp_x16 loop in intel_dp_mtp_tu_compute_config() in intel_dp_mst.c?

Because intel_dp_check_m_n_ratio() uses drm_WARN() and is called inside this
loop, a KMS client requesting an MST mode where the link_m / link_n ratio
exceeds max_m_n_ratio could trigger a full kernel stack trace for every failed
loop iteration.

When DSC is used, bpp_step_x16 can be as small as 1/16 bpp. Could this cause
the loop to run potentially hundreds of times for an un-fittable mode, flooding
the kernel log and leading to dmesg rollover or console lockups?

> +
>  		if (is_mst) {
>  			int remote_bw_overhead;
>  			int remote_tu;

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Sashiko AI review · https://sashiko.dev/#/patchset/20260916090946.677564-1-suraj.kandpal@intel.com?part=4