[Intel-gfx] [PATCH 0/5] Handle Link Training Failure during modeset

[Cheng, Tony]

Amdgpu dal implementation will do a test link training at end of detection to verify we can achieve the capability reported in DPCD.  We then report mode base on result of test training.

AMD hardware (at least the generations supported by amdgpu) is able to link train without timing being setup (DP encoder and CRTC is decoupled).  Do we have limitation from other vendors where you need timing to be there before you can link train?

We can also past DP1.2 link layer compliance with this approach.

-----Original Message-----
From: Deucher, Alexander 
Sent: Thursday, November 10, 2016 1:43 PM
To: 'Jani Nikula' <jani.nikula at linux.intel.com>; Manasi Navare <manasi.d.navare at intel.com>; dri-devel at lists.freedesktop.org; intel-gfx at lists.freedesktop.org; Wentland, Harry <Harry.Wentland at amd.com>; Cheng, Tony <Tony.Cheng at amd.com>
Cc: Dave Airlie <airlied at gmail.com>; Peres, Martin <martin.peres at intel.com>
Subject: RE: [Intel-gfx] [PATCH 0/5] Handle Link Training Failure during modeset

Adding Harry and Tony from our display team to review.

> -----Original Message-----
> From: Jani Nikula [mailto:jani.nikula at linux.intel.com]
> Sent: Thursday, November 10, 2016 1:20 PM
> To: Manasi Navare; dri-devel at lists.freedesktop.org; intel- 
> gfx at lists.freedesktop.org
> Cc: Dave Airlie; Peres, Martin; Deucher, Alexander
> Subject: Re: [Intel-gfx] [PATCH 0/5] Handle Link Training Failure 
> during modeset
> 
> On Thu, 10 Nov 2016, Manasi Navare <manasi.d.navare at intel.com> wrote:
> > Link training failure is handled by lowering the link rate first 
> > until it reaches the minimum and keeping the lane count maximum and 
> > then lowering the lane count until it reaches minimim. These 
> > fallback values are saved and hotplug uevent is sent to the 
> > userspace after setting the connector link status property to BAD. 
> > Userspace should triiger another modeset on a uevent and if link 
> > status property is BAD. This will retrain the link at fallback values.
> > This is repeated until the link is successfully trained.
> >
> > This has been validated to pass DP compliance.
> 
> This cover letter and the commit messages do a good job of explaining 
> what the patches do. However, you're lacking the crucial information 
> of
> *why* we need userspace cooperation to handle link training failures 
> on DP mode setting, and *why* a new connector property is a good 
> solution for this.
> 
> Here goes, including some alternative approaches we've considered (and 
> even tried):
> 
> At the time userspace does setcrtc, we've already promised the mode 
> would work. The promise is based on the theoretical capabilities of 
> the link, but it's possible we can't reach this in practice. The DP 
> spec describes how the link should be reduced, but we can't reduce the 
> link below the requirements of the mode. Black screen follows.
> 
> One idea would be to have setcrtc return a failure. However, it is my 
> understanding that it already should not fail as the atomic checks 
> have passed [citation needed]. It would also conflict with the idea of 
> making setcrtc asynchronous in the future, returning before the actual 
> mode setting and link training.
> 
> Another idea is to train the link "upfront" at hotplug time, before 
> pruning the mode list, so that we can do the pruning based on 
> practical not theoretical capabilities. However, the changes for link 
> training are pretty drastic, all for the sake of error handling and DP 
> compliance, when the most common happy day scenario is the current 
> approach of link training at mode setting time, using the optimal 
> parameters for the mode. It is also not certain all hardware could do 
> this without the pipe on; not even all our hardware can do this. Some 
> of this can be solved, but not trivially.
> 
> Both of the above ideas also fail to address link degradation *during* 
> operation.
> 
> So the idea presented in these patches is to do this in a way that a) 
> changes the current happy day scenario as little as possible, to avoid 
> regressions, b) can be implemented the same way by all drm drivers, c) 
> is still opt-in for the drivers and userspace, and opting out doesn't 
> regress the user experience, d) doesn't prevent drivers from 
> implementing better or alternate approaches, possibly without 
> userspace involvement. And, of course, handles all the issues presented.
> 
> The solution is to add a "link status" connector property. In the 
> usual happy day scenario, this is always "good". If something fails 
> during or after a mode set, the kernel driver can set the link status 
> to "bad", prune the mode list based on new information as necessary, 
> and send a hotplug uevent for userspace to have it re-check the valid 
> modes through getconnector, and try again. If the theoretical 
> capabilities of the link can't be reached, the mode list is trimmed based on that.
> 
> If the userspace is not aware of the property, the user experience is 
> the same as it currently is. If the userspace is aware of the 
> property, it has a chance to improve user experience. If a drm driver 
> does not modify the property (it stays "good"), the user experience is 
> the same as it currently is. A drm driver can also choose to try to 
> handle more of the failures in kernel, hardware not limiting, or it 
> can choose to involve userspace more. Up to the drivers.
> 
> The reason for adding the property is to handle link training 
> failures, but it is not limited to DP or link training. For example, 
> if we implement asynchronous setcrtc, we can use this to report any 
> failures in that.
> 
> Finally, while DP CTS compliance is advertized (which is great, and 
> could be made to work similarly for all drm drivers), this can be used 
> for the more important goal of improving user experience on link 
> training failures, by avoiding black screens.
> 
> 
> BR,
> Jani.
> 
> 
> >
> > Manasi Navare (5):
> >   drm: Add a new connector property for link status
> >   drm/i915: Set link status property for DP connector
> >   drm/i915: Update CRTC state if connector link status property 
> > changed
>     ^^^^^^^^
> 
> This is really drm, not i915.
> 
> 
> >   drm/i915: Find fallback link rate/lane count
> >   drm/i915: Implement Link Rate fallback on Link training failure
> >
> >  drivers/gpu/drm/drm_atomic_helper.c           |   7 ++
> >  drivers/gpu/drm/drm_connector.c               |  17 ++++
> >  drivers/gpu/drm/i915/intel_ddi.c              |  21 +++-
> >  drivers/gpu/drm/i915/intel_dp.c               | 138
> +++++++++++++++++++++++++-
> >  drivers/gpu/drm/i915/intel_dp_link_training.c |  12 ++-
> >  drivers/gpu/drm/i915/intel_drv.h              |  12 ++-
> >  include/drm/drm_connector.h                   |   7 +-
> >  include/drm/drm_crtc.h                        |   5 +
> >  include/uapi/drm/drm_mode.h                   |   4 +
> >  9 files changed, 214 insertions(+), 9 deletions(-)
> 
> --
> Jani Nikula, Intel Open Source Technology Center