[PATCH v3] drm/amd/display: Fix AppleDongle can't be detected

[Louis Li]

On Tue, Dec 17, 2019 at 10:57:11AM -0500, Harry Wentland wrote:
> On 2019-12-11 2:33 a.m., Louis Li wrote:
> > [Why]
> > External monitor cannot be displayed consistently, if connecting
> > via this Apple dongle (A1621, USB Type-C to HDMI).
> > Experiments prove that the dongle needs 200ms at least to be ready
> > for communication, after it drives HPDsignal high, and DPCD cannot
> > be read correctly during the period, even reading it repeatedly.
> > In such a case, driver does not perform link training bcz of no DPCD.
> > 
> > [How]
> > When driver is run to the modified point, EDID is read correctly
> > and dpcd_sink_count of link is not zero. Therefore, link training
> > should be successfully performed. Which implies parameters should
> > be updated, e.g. lane count, link rate, etc. Checking parameters,
> > if values of those parameters are zero, link training is not
> > performed. So, do link-training to have detection completed.
> > 
> > With this patch applied, the problem cannot be reproduced.
> > Testing other dongles, results are PASS.
> > Patch(v3) is verified PASS by both AMD internal lab and customer.
> > 
> > 
> > Signed-off-by: Louis Li <Ching-shih.Li at amd.com>
> > ---
> >  drivers/gpu/drm/amd/display/dc/core/dc_link.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > 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 7372dedd2f48..6188edc92d0f 100644
> > --- a/drivers/gpu/drm/amd/display/dc/core/dc_link.c
> > +++ b/drivers/gpu/drm/amd/display/dc/core/dc_link.c
> > @@ -725,7 +725,9 @@ bool dc_link_detect(struct dc_link *link, enum dc_detect_reason reason)
> >  
> >  		if (link->connector_signal == SIGNAL_TYPE_DISPLAY_PORT &&
> >  			sink_caps.transaction_type == DDC_TRANSACTION_TYPE_I2C_OVER_AUX &&
> > -			reason != DETECT_REASON_HPDRX) {
> 
> Do we need to drop this line? This looks like it'll break the previous
> fix here.
> 
> It looks like Abdoulaye added this here to fix the 400.1.1 DP compliance
> test. If you can check with him that your solution is fine and make sure
> to test that you can get a consistent pass of 400.1.1 over 30 runs I'm
> okay to take the change.
> 
> Harry
> 

Yes, need drop this line for this fix. Good to know it may impact 400.1.1.
I will verify it with this patch. And update test result.

Louis

> > +			link->verified_link_cap.lane_count == 0 &&
> > +			link->verified_link_cap.link_rate == 0 &&
> > +			link->verified_link_cap.link_spread == 0) {
> >  			/*
> >  			 * TODO debug why Dell 2413 doesn't like
> >  			 *  two link trainings
> >