Re: [PATCH i-g-t v1 1/2] lib/igt_kms: Add MST bandwidth-fitting support and helpers

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

On Tue, 21 Apr 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.
>
> Signed-off-by: Sowmiya S <sowmiya.s@intel.com>
> ---
>  lib/igt_kms.c | 33 +++++++++++++++++++++++++++++++++
>  lib/igt_kms.h |  3 +++
>  2 files changed, 36 insertions(+)
>
> diff --git a/lib/igt_kms.c b/lib/igt_kms.c
> index 2d758b63c..0c7965954 100644
> --- a/lib/igt_kms.c
> +++ b/lib/igt_kms.c
> @@ -2744,6 +2744,7 @@ static void igt_output_reset(igt_output_t *output)
>  {
>  	output->pending_crtc = NULL;
>  	output->use_override_mode = false;
> +	output->bw_fit_applied = false;
>  	memset(&output->override_mode, 0, sizeof(output->override_mode));
>  
>  	igt_output_set_prop_value(output, IGT_CONNECTOR_CRTC_ID, 0);
> @@ -3227,6 +3228,15 @@ void igt_display_require(igt_display_t *display, int drm_fd)
>  
>  	igt_display_reset_outputs(display);
>  
> +	/* Detect MST outputs after they are fully initialized */
> +	for (i = 0; i < display->n_outputs; i++) {

for_each_output()

> +		if (display->outputs[i].config.connector &&
> +		    igt_check_output_is_dp_mst(&display->outputs[i])) {
> +			display->needs_bw_fit = true;

So that really means "has DP MST output"? Why not name it according to
what it actually *is*, not what it's used for? Or add a function to
check for that instead of caching it and dealing with all the troubles
of caching.

> +			break;
> +		}
> +	}
> +
>  out:
>  	LOG_UNINDENT(display);
>  
> @@ -5372,6 +5382,7 @@ bool __override_all_active_output_modes_to_fit_bw(igt_display_t *display,
>  		int ret;
>  
>  		igt_output_override_mode(output, mode);
> +		output->bw_fit_applied = true;
>  
>  		if (__override_all_active_output_modes_to_fit_bw(display, outputs, n_outputs, base + 1))
>  			return true;
> @@ -5441,6 +5452,9 @@ bool igt_fit_modes_in_bw(igt_display_t *display)
>  {
>  	int ret;
>  
> +	for (int i = 0; i < display->n_outputs; i++)
> +		display->outputs[i].bw_fit_applied = false;

for_each_output()

> +
>  	if (display->is_atomic)
>  		ret = igt_display_try_commit_atomic(display,
>  						    DRM_MODE_ATOMIC_TEST_ONLY |
> @@ -5461,6 +5475,25 @@ bool igt_fit_modes_in_bw(igt_display_t *display)
>  	return true;
>  }
>  
> +void igt_display_fit_bw(igt_display_t *display)

I probably haven't always documented every function I've added, but
things like igt_crtc_for_crtc_index() are pretty obvious.

I have no idea what this actually does.

> +{
> +	int active_mst = 0;
> +
> +	if (!display->needs_bw_fit)
> +		return;
> +	for (int i = 0; i < display->n_outputs; i++) {

for_each_output()

> +		igt_output_t *output = &display->outputs[i];
> +
> +		if (output->pending_crtc &&
> +		    igt_check_output_is_dp_mst(output))
> +			active_mst++;
> +	}
> +	if (active_mst < 2)
> +		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 fcbb6a5ad..5c60e7bb4 100644
> --- a/lib/igt_kms.h
> +++ b/lib/igt_kms.h
> @@ -506,6 +506,7 @@ typedef struct {
>  
>  	/* bitmask of changed properties */
>  	uint64_t changed;
> +	bool bw_fit_applied;
>  
>  	uint32_t props[IGT_NUM_CONNECTOR_PROPS];
>  	uint64_t values[IGT_NUM_CONNECTOR_PROPS];
> @@ -536,6 +537,7 @@ struct _igt_display {
>  	uint64_t *modifiers;
>  	uint32_t *formats;
>  	int format_mod_count;
> +	bool needs_bw_fit;
>  };
>  
>  typedef struct {
> @@ -1286,6 +1288,7 @@ 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_display_fit_bw(igt_display_t *display);
>  
>  igt_colorop_t *igt_find_colorop(igt_display_t *display, uint32_t id);

-- 
Jani Nikula, Intel