Re: [PATCH 2/2] drm/amd/display: Allow current eDP link settings to override verified ones.

[Harry Wentland <hwentlan@amd.com>]

On 2020-01-09 10:20 a.m., Mario Kleiner wrote:
> If the current eDP link settings, as read from hw, provide a higher
> bandwidth than the verified_link_cap ones (= reported_link_cap), then
> override verified_link_cap with current settings.
> 
> These initial current eDP link settings have been set up by
> firmware during boot, so they should work on the eDP panel.
> Therefore use them if the firmware thinks they are good and
> they provide higher link bandwidth, e.g., to enable higher
> resolutions / color depths.
> 

This only works when taking over from UEFI, so on boot or resume from
hibernate. This wouldn't work on a normal suspend/resume.

Can you check if setting link->dc->config.optimize_edp_link_rate (see
first if statement in detect_edp_sink_caps) fixes this? I imagine we
need to read the reported settings from DP_SUPPORTED_LINK_RATES and fail
to do so.

Thanks,
Harry

> This fixes a problem found on the MacBookPro 2017 Retina panel:
> 
> The panel reports 10 bpc color depth in its EDID, and the
> firmware chooses link settings at boot which support enough
> bandwidth for 10 bpc (324000 kbit/sec aka LINK_RATE_RBR2),
> but the DP_MAX_LINK_RATE dpcd register only reports 2.7 Gbps
> as possible, so verified_link_cap is only good for 2.7 Gbps
> and 8 bpc, not providing the full color depth of the panel.
> 
> Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
> Cc: Alex Deucher <alexander.deucher@amd.com>
> ---
>  drivers/gpu/drm/amd/display/dc/core/dc_link.c | 21 +++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/drivers/gpu/drm/amd/display/dc/core/dc_link.c b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
> index 5ea4a1675259..f3acdb8fead5 100644
> --- a/drivers/gpu/drm/amd/display/dc/core/dc_link.c
> +++ b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
> @@ -819,6 +819,27 @@ static bool dc_link_detect_helper(struct dc_link *link,
>  		case SIGNAL_TYPE_EDP: {
>  			detect_edp_sink_caps(link);
>  			read_current_link_settings_on_detect(link);
> +
> +			/* If cur_link_settings provides higher bandwidth than
> +			 * verified_link_cap, then use cur_link_settings as new
> +			 * verified_link_cap, as it obviously works according to
> +			 * firmware boot setup.
> +			 *
> +			 * This has been observed on the Apple MacBookPro 2017
> +			 * Retina panel, which boots with a link setting higher
> +			 * than what dpcd[DP_MAX_LINK_RATE] claims as possible.
> +			 * Overriding allows to run the panel at 10 bpc / 30 bit.
> +			 */
> +			if (dc_link_bandwidth_kbps(link, &link->cur_link_settings) >
> +			    dc_link_bandwidth_kbps(link, &link->verified_link_cap)) {
> +				DC_LOG_DETECTION_DP_CAPS(
> +				"eDP current link setting bw %d kbps > verified_link_cap %d kbps. Override.",
> +				dc_link_bandwidth_kbps(link, &link->cur_link_settings),
> +				dc_link_bandwidth_kbps(link, &link->verified_link_cap));
> +
> +				link->verified_link_cap = link->cur_link_settings;
> +			}
> +
>  			sink_caps.transaction_type = DDC_TRANSACTION_TYPE_I2C_OVER_AUX;
>  			sink_caps.signal = SIGNAL_TYPE_EDP;
>  			break;
> 
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