[PATCH] drm/i915/dp: Return early if dsc is required but not supported

[Jani Nikula]

On Fri, 03 Jan 2025, Ankit Nautiyal <ankit.k.nautiyal at intel.com> wrote:
> Currently, when bandwidth is insufficient for a given mode, we attempt
> to use DSC. This is indicated by a debug print, followed by a check for
> DSC support.
>
> The debug message states that we are trying DSC, but DSC might not be
> supported, which can give an incorrect picture in the logs if we bail
> out later.
>
> Correct the order for both DP and DP MST to:
> - Check if DSC is required and supported, and return early if DSC is
> not supported.
> - Print a debug message to indicate that DSC will be tried next.
>
> Suggested-by: Jani Nikula <jani.nikula at linux.intel.com>
> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal at intel.com>

Reviewed-by: Jani Nikula <jani.nikula at intel.com>

> ---
>  drivers/gpu/drm/i915/display/intel_dp.c     | 8 +++++---
>  drivers/gpu/drm/i915/display/intel_dp_mst.c | 7 +++++--
>  2 files changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 0d74adae2ec9..4fa0e0b675b0 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -2642,15 +2642,17 @@ intel_dp_compute_link_config(struct intel_encoder *encoder,
>  			dsc_needed = true;
>  	}
>  
> +	if (dsc_needed && !intel_dp_supports_dsc(intel_dp, connector, pipe_config)) {
> +		drm_dbg_kms(display->drm, "DSC required but not available\n");
> +		return -EINVAL;
> +	}
> +
>  	if (dsc_needed) {
>  		drm_dbg_kms(display->drm,
>  			    "Try DSC (fallback=%s, joiner=%s, force=%s)\n",
>  			    str_yes_no(ret), str_yes_no(joiner_needs_dsc),
>  			    str_yes_no(intel_dp->force_dsc_en));
>  
> -		if (!intel_dp_supports_dsc(intel_dp, connector, pipe_config))
> -			return -EINVAL;
> -
>  		if (!intel_dp_compute_config_limits(intel_dp, pipe_config,
>  						    respect_downstream_limits,
>  						    true,
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index fffd199999e0..0433f2ff77e1 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -633,14 +633,17 @@ static int mst_stream_compute_config(struct intel_encoder *encoder,
>  			dsc_needed = true;
>  	}
>  
> +	if (dsc_needed && !intel_dp_supports_dsc(intel_dp, connector, pipe_config)) {
> +		drm_dbg_kms(display->drm, "DSC required but not available\n");
> +		return -EINVAL;
> +	}
> +
>  	/* enable compression if the mode doesn't fit available BW */
>  	if (dsc_needed) {
>  		drm_dbg_kms(display->drm, "Try DSC (fallback=%s, joiner=%s, force=%s)\n",
>  			    str_yes_no(ret), str_yes_no(joiner_needs_dsc),
>  			    str_yes_no(intel_dp->force_dsc_en));
>  
> -		if (!intel_dp_supports_dsc(intel_dp, connector, pipe_config))
> -			return -EINVAL;
>  
>  		if (!mst_stream_compute_config_limits(intel_dp, connector,
>  						      pipe_config, true,

-- 
Jani Nikula, Intel