Re: [PATCH v8 02/14] drm/i915/dp: Limit M/N ratio for DP

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] drm/i915/dp: intel_dp_test_compute_config() clobbers M/N ratio filter
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commit 555420da2bc55656b200265b485cbd9bd62d516b
Author: Suraj Kandpal <suraj.kandpal@intel.com>

drm/i915/dp: Limit M/N ratio for DP

This commit restricts DisplayPort link configurations to those where the
ceiling of the Link M/N ratio does not exceed the hardware limit (4 on DG2,
10 on other platforms), enforcing the constraint both in mode_valid and
when computing link configuration limits.

> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index d8c0d9381471d..661a863362ad4 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c

[ ... ]

> @@ -2772,6 +2886,9 @@ intel_dp_compute_config_limits(struct intel_dp *intel_dp,
>  		limits->link_config_filter = new_filter;
>  	}
>  
> +	if (!intel_dp_compute_m_n_ratio_limits(intel_dp, conn_state, crtc_state, limits))
> +		return false;
> +
>  	if (!intel_dp_test_compute_config(connector, crtc_state, limits))
>  		return false;

[Severity: Medium]
Can intel_dp_test_compute_config() clobber the M/N ratio filter that was just
computed by intel_dp_compute_m_n_ratio_limits()?

When intel_dp->compliance.test_type == DP_TEST_LINK_TRAINING,
intel_dp_test_compute_config() calls set_filter_for_link_params() ->
set_filter_for_link_config() in drivers/gpu/drm/i915/display/intel_dp_test.c:

static bool set_filter_for_link_config(struct intel_connector *connector,
				       struct intel_dp_link_caps *link_caps,
				       const struct intel_dp_link_config *link_params,
				       struct link_config_limits *limits)
{
	struct intel_dp_link_caps_filter new_filter = INTEL_DP_LINK_CAPS_FILTER_NONE;

	if (!intel_dp_link_caps_filter_add(link_caps, &new_filter, link_params))
		return false;

	limits->link_config_filter = new_filter;

	return true;
}

Because intel_dp_link_caps_filter_add() only checks
get_allowed_config_filter(link_caps) and ignores limits->link_config_filter,
set_filter_for_link_config() overwrites limits->link_config_filter with a
fresh filter containing the compliance test link configuration even if that
link rate was filtered out by intel_dp_compute_m_n_ratio_limits().

Should intel_dp_compute_m_n_ratio_limits() be called after
intel_dp_test_compute_config(), or should set_filter_for_link_config()
intersect against the existing limits->link_config_filter?

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Sashiko AI review · https://sashiko.dev/#/patchset/20261023050227.2068466-1-suraj.kandpal@intel.com?part=2