Re: [PATCH i-g-t v4 1/3] lib/igt_kms: Add MST bandwidth-fitting support and helpers

[Jani Nikula <jani.nikula@intel.com>]

On Mon, 29 Jun 2026, Sowmiya S <sowmiya.s@intel.com> wrote:
> Add helpers to handle shared-link DP bandwidth fitting for
> MST/daisy-chain configurations.
>
> On MST hubs/daisy-chains multiple streams share one DP link
> and a joint atomic commit can be rejected unless modes are
> chosen to fit the link BW. This will centralizes MST detection
> and fitting so tests can request fitting only when required.
>
> v2: Remove the flag used for caching values and update the
> function name to better describe its functionality (Jani)
> v3: Apply mst bw fit for shared link (Kunal)
>     Remove unused variable (Kunal)
>
> Signed-off-by: Sowmiya S <sowmiya.s@intel.com>
> ---
>  lib/igt_kms.c | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  lib/igt_kms.h |  1 +
>  2 files changed, 75 insertions(+)
>
> diff --git a/lib/igt_kms.c b/lib/igt_kms.c
> index e82d32130..fceca2a1b 100644
> --- a/lib/igt_kms.c
> +++ b/lib/igt_kms.c
> @@ -5472,6 +5472,80 @@ bool igt_fit_modes_in_bw(igt_display_t *display)
>  	return true;
>  }
>  
> +/**
> + * igt_outputs_share_mst_link:
> + * @output_a: first output
> + * @output_b: second output
> + *
> + * Returns: true if both outputs are active DP-MST streams that share the
> + * same upstream link, i.e. their root connector IDs (as encoded in the
> + * MST PATH property) are equal.  Returns false if either output is not an
> + * active MST stream or if they are rooted at different connectors.
> + */
> +static bool igt_outputs_share_mst_link(igt_output_t *output_a,
> +				       igt_output_t *output_b)
> +{
> +	if (!output_a->pending_crtc || !igt_check_output_is_dp_mst(output_a))
> +		return false;

The pending_crtc member is supposed to be private to igt_output_t. You
can use igt_output_get_driving_crtc(). Ditto everywhere.

> +
> +	if (!output_b->pending_crtc || !igt_check_output_is_dp_mst(output_b))
> +		return false;
> +
> +	return igt_get_dp_mst_connector_id(output_a) ==
> +	       igt_get_dp_mst_connector_id(output_b);
> +}
> +
> +/**
> + * igt_require_mst_link_bw:
> + * @display: a pointer to an #igt_display_t structure
> + *
> + * When two or more DP-MST streams share the same upstream link (i.e. they
> + * are both rooted at the same MST hub), a joint atomic commit may be
> + * rejected by the driver if the combined stream bandwidth exceeds the link
> + * capacity.  This function detects that situation by comparing the root
> + * connector ID embedded in each output's MST PATH property and, when two
> + * or more active outputs share the same root, calls igt_fit_modes_in_bw()
> + * to select mode overrides that satisfy the shared-link BW budget.  The
> + * test is skipped if no valid combination can be found.
> + *
> + * TODO: Extend this function to proactively detect outputs that share a
> + * DP tunnel (USB4/Thunderbolt) and trigger igt_fit_modes_in_bw() for
> + * those too.
> + */
> +void igt_require_mst_link_bw(igt_display_t *display)
> +{
> +	igt_output_t *mst_outputs[IGT_MAX_PIPES];

I've seen the mistake elsewhere, but IGT_MAX_PIPES is not the same as
the max number of outputs (which is based on connectors).

> +	int n_mst = 0;
> +	igt_output_t *output;
> +	bool shared_link = false;
> +	int i, j;
> +
> +	/* Collect all active MST outputs in one pass. */
> +	for_each_output(display, output) {
> +		if (output->pending_crtc && igt_check_output_is_dp_mst(output))
> +			mst_outputs[n_mst++] = output;

So this could overflow.

> +	}
> +
> +	/* Need at least two MST outputs before a shared-link is possible. */
> +	if (n_mst < 2)
> +		return;
> +
> +	/* Check every pair for a shared upstream root connector. */
> +	for (i = 0; i < n_mst && !shared_link; i++)
> +		for (j = i + 1; j < n_mst; j++)
> +			if (igt_outputs_share_mst_link(mst_outputs[i],
> +						       mst_outputs[j])) {
> +				shared_link = true;
> +				break;
> +			}
> +
> +	if (!shared_link)
> +		return;
> +
> +	igt_require_f(igt_fit_modes_in_bw(display),
> +		      "No valid mode combination fits available MST link BW\n");
> +}
> +
>  /**
>   * igt_crtc_refresh:
>   * @crtc: CRTC to refresh
> diff --git a/lib/igt_kms.h b/lib/igt_kms.h
> index 3df3ba866..f5aff5008 100644
> --- a/lib/igt_kms.h
> +++ b/lib/igt_kms.h
> @@ -1370,6 +1370,7 @@ drmModePropertyBlobRes *igt_get_writeback_formats_blob(igt_output_t *output);
>  uint64_t igt_get_writeback_fb_id(igt_output_t *output);
>  void igt_detach_crtc(igt_display_t *display, igt_output_t *output);
>  void igt_get_and_wait_out_fence(igt_output_t *output);
> +void igt_require_mst_link_bw(igt_display_t *display);
>  
>  igt_colorop_t *igt_find_colorop(igt_display_t *display, uint32_t id);

-- 
Jani Nikula, Intel