[Intel-gfx] [PATCH v7 1/6] drm/i915: Fallback to lower link rate and lane count during link training

[Chris Wilson]

On Thu, Sep 29, 2016 at 02:26:16PM +0300, Jani Nikula wrote:
> On Thu, 29 Sep 2016, Manasi Navare <manasi.d.navare at intel.com> wrote:
> > On Tue, Sep 27, 2016 at 08:07:01PM +0300, Jani Nikula wrote:
> >> On Tue, 27 Sep 2016, Manasi Navare <manasi.d.navare at intel.com> wrote:
> >> > On Mon, Sep 26, 2016 at 04:39:34PM +0300, Jani Nikula wrote:
> >> >> On Fri, 16 Sep 2016, Manasi Navare <manasi.d.navare at intel.com> wrote:
> >> >> > According to the DisplayPort Spec, in case of Clock Recovery failure
> >> >> > the link training sequence should fall back to the lower link rate
> >> >> > followed by lower lane count until CR succeeds.
> >> >> > On CR success, the sequence proceeds with Channel EQ.
> >> >> > In case of Channel EQ failures, it should fallback to
> >> >> > lower link rate and lane count and start the CR phase again.
> >> >> 
> >> >> This change makes the link training start at the max lane count and max
> >> >> link rate. This is not ideal, as it wastes the link. And it is not a
> >> >> spec requirement. "The Link Policy Maker of the upstream device may
> >> >> choose any link count and link rate as long as they do not exceed the
> >> >> capabilities of the DP receiver."
> >> >> 
> >> >> Our current code starts at the minimum required bandwidth for the mode,
> >> >> therefore we can't fall back to lower link rate and lane count without
> >> >> reducing the mode.
> >> >> 
> >> >> AFAICT this patch here makes it possible for the link bandwidth to drop
> >> >> below what is required for the mode. This is unacceptable.
> >> >> 
> >> >> BR,
> >> >> Jani.
> >> >> 
> >> >>
> >> >
> >> > Thanks Jani for your review comments.
> >> > Yes in this change we start at the max link rate and lane count. This
> >> > change was made according to the design document discussions we had
> >> > before strating this DP Redesign project. The main reason for starting
> >> > at the maxlink rate and max lane count was for ensuring proper
> >> > behavior of DP MST. In case of DP MST, we want to train the link at
> >> > the maximum supported link rate/lane count based on an early/ upfront
> >> > link training result so that we dont fail when we try to connect a
> >> > higher resolution monitor as a second monitor. This a trade off
> >> > between wsting the link or higher power vs. needing to retrain for
> >> > every monitor that requests a higher BW in case of DP MST.
> >> 
> >> We already train at max bandwidth for DP MST, which seems to be the
> >> sensible thing to do.
> >> 
> >> > Actually this is also the reason for enabling upfront link training in
> >> > the following patch where we train the link much ahead in the modeset
> >> > sequence to understand the link rate and lane count values at which
> >> > the link can be successfully trained and then the link training
> >> > through modeset will always start at the upfront values (maximum
> >> > supported values of lane count and link rate based on upfront link
> >> > training).
> >> 
> >> I don't see a need to do this for DP SST.
> >> 
> >> > As per the CTS, all the test 4.3.1.4 requires that you fall back to
> >> > the lower link rate after trying to train at the maximum link rate
> >> > advertised through the DPCD registers.
> >> 
> >> That test does not require the source DUT to default to maximum lane
> >> count or link rate of the sink. The source may freely choose the lane
> >> count and link rate as long as they don't exceed sink capabilities.
> >> 
> >> For the purposes of the test, the test setup can request specific
> >> parameters to be used, but that does not mean using maximum by
> >> *default*.
> >> 
> >> We currently lack the feature to reduce lane count and link rate. The
> >> key to understand here is that starting at max and reducing down to the
> >> sufficient parameters for the mode (which is where we start now) offers
> >> no real benefit for any use case. What we're lacking is a feature to
> >> reduce the link parameters *below* what's required by the mode the
> >> userspace wants. This can only be achieved through cooperation with
> >> userspace.
> >> 
> >
> > We can train at the optimal link rate required for the requested mode as
> > done in the existing implementation and retrain whenever the link training
> > test request is sent. 
> > For the test 4.3.1.4 in CTS, it does force a failure in CR and expects the
> > driver to fall back to even lower link rate. We do not implement this in the
> > current driver and so this test fails. Could you elaborate on how this can
> > be achieved with the the cooperation with userspace?
> > Should we send a uevent to the userspace asking to retry at a lower resolution
> > after retraining at the lower link rate?
> > This is pertty much the place where majority of the compliance tests are failing.
> > How can we pass compliance with regards to this feature?
> 
> So here's an idea Ville and I came up with. It's not completely thought
> out yet, probably has some wrinkles still, but then there are wrinkles
> with the upfront link training too (I'll get back to those separately).
> 
> If link training fails during modeset (either for real or because it's a
> test sink that wants to test failures), we 1) store the link parameters
> as failing, 2) send a uevent to userspace, hopefully getting the
> userspace to do another get modes and try again, 3) propage errors from
> modeset.

userspace already tries to do a reprobe after a setcrtc fails, to try
and gracefully handle the race between hotplug being in its event queue
and performing setcrtc, i.e. I think the error is enough.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre