Re: [PATCH 03/10] drm/i915/dp_mst: Limit M/N ratio for MST

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The M/N ratio check in mst_connector_mode_valid_ctx uses the link rate of the max bandwidth configuration, falsely rejecting valid display modes that could be supported by a narrower but faster configuration.
--

commit 98416996dd21ed919f77966254ec5cd4baea6754
Author: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
drm/i915/dp_mst: Limit M/N ratio for MST

This commit adds M/N ratio limits for DisplayPort MST configurations by 
pruning modes in mst_connector_mode_valid_ctx() that exceed hardware 
limits. It mirrors the SST mode validation logic.

> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index 6a869d0f6ffc0..796374cebf051 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
[ ... ]
> @@ -1564,6 +1564,11 @@ mst_connector_mode_valid_ctx(struct drm_connector *_connector,
>  					   min_link_bpp_x16,
>  					   bw_overhead_flags);
>  
> +	if (!intel_dp_can_support_m_n(display, mode->clock, max_link_clock)) {
> +		*status = MODE_CLOCK_HIGH;
> +		return 0;
> +	}
> +

[Severity: Medium]
The commit message states this check is added "using the highest link rate 
available", mirroring the SST mode_valid check. However, max_link_clock 
here is derived from max_bw_config.

Could this falsely reject valid display modes if the link capabilities 
result in a max bandwidth configuration with a lower symbol clock than an 
alternative configuration? 

For example, if max_bw_config falls back to RBR x 4 (162MHz) but an 
alternative HBR x 2 (270MHz) configuration is available, a mode's pixel 
clock might exceed the M/N limit for the 162MHz config but safely fit 
within the bandwidth and M/N limits of the 270MHz configuration. Does this 
logic prematurely restrict those modes to the M/N ratio of the max 
bandwidth config?

>  	/*
>  	 * TODO:
>  	 * - Also check if compression would allow for the mode

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260911041449.3699400-1-suraj.kandpal@intel.com?part=3