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

[Harry Wentland]

On 2020-01-09 4:13 p.m., Mario Kleiner wrote:
> On Thu, Jan 9, 2020 at 7:44 PM Harry Wentland <hwentlan at amd.com
> <mailto:hwentlan at amd.com>> wrote:
> 
>     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.
>     >
>      
> 
> 
> Hi Harry, happy new year!
> 

Frohes Neues. :)

>     This only works when taking over from UEFI, so on boot or resume from
>     hibernate. This wouldn't work on a normal suspend/resume.
> 
> 
> See the other thread i just cc'ed you on. Depends if
> dc_link_detect_helper() gets skipped/early returns or not on EDP. Some
> if statement suggests it might get skipped on EDP + resume?
>  

You've likely looked at the code more closely while debugging this than
I have recently. It looks like we indeed skip detection if we've
previously detected the eDP sink.

> 
>     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.
> 
> 
> Tried that already (see other mail), replacing the whole if statement
> with a if (true) to force reading DP_SUPPORTED_LINK_RATES. The whole
> table reads back as all-zero, and versions are DP 1.1, eDP 1.3, not 1.4+
> as what seems to be required. The use the classic link bw stuff, but
> with a non-standard link bandwidth multiplier of 0xc, and a reported
> DP_MAX_LINK_RATE of 0xa, contradicting the 0xc setting that the firmware
> sets at bootup.
> 
> Seems to be a very Apple thing...

Indeed. I think it was a funky panel that was "ahead of its time" and
ahead of the spec.

I would prefer a DPCD quirk for this panel that updates the reported DP
caps, rather than picking the "current" ones from the FW lightup.

Harry

> -mario
>  
> 
> 
>     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 at gmail.com
>     <mailto:mario.kleiner.de at gmail.com>>
>     > Cc: Alex Deucher <alexander.deucher at amd.com
>     <mailto:alexander.deucher at 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;
>     >
>