[Bug 91871] link retraining for DP not possible with current design of Atomic modeset framework

[Bug 91871] link retraining for DP not possible with current design of Atomic modeset framework

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https://bugs.freedesktop.org/show_bug.cgi?id=91871

            Bug ID: 91871
           Summary: link retraining for DP not possible with current
                    design of Atomic modeset framework
           Product: DRI
           Version: XOrg git
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: General
          Assignee: dri-devel@lists.freedesktop.org
          Reporter: sivakumar.thulasimani@intel.com

DP spec requires Link training from highest link rate supported by panel to
lowest link rate supported by panel. 

It is not possible to implement this in the current atomic modeset logic since
it splits modeset into two half atomic_check and atomic_commit, where it is
expected all values to be calculated in atomic_check and programmed in
atomic_commit. 

link training being a negotiation phase of DP modeset it is not possible to
predict if a training at certain link rate will succeed or not. if it does fail
it will require us to disable, calculate new values and enable pll. this is not
possible in current DRM Atomic modeset framework.

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[Bug 91871] link retraining for DP not possible with current design of Atomic modeset framework

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https://bugs.freedesktop.org/show_bug.cgi?id=91871

rtshiva <sivakumar.thulasimani@intel.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|XOrg git                    |unspecified

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[Bug 91871] link retraining for DP not possible with current design of Atomic modeset framework

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https://bugs.freedesktop.org/show_bug.cgi?id=91871

Jani Nikula <jani.nikula@intel.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |conselvan2@gmail.com,
                   |                            |intel-gfx-bugs@lists.freede
                   |                            |sktop.org
          Component|General                     |DRM/Intel
           Assignee|dri-devel@lists.freedesktop |intel-gfx-bugs@lists.freede
                   |.org                        |sktop.org
         QA Contact|                            |intel-gfx-bugs@lists.freede
                   |                            |sktop.org

--- Comment #1 from Jani Nikula <jani.nikula@intel.com> ---
To be fair, the same limitation was there before atomic.

Part of the problem is our modeset sequence, which we can't easily backtrack if
the modeset fails at link training phase. Another part is that even if our
modeset sequence supported that, we couldn't just reduce the mode to
accommodate the lower link rate behind userspace's back. (We already pick the
lowest rate that can support the mode; there's no margin to reduce.) We'd have
to involve userspace somehow.

Drivers are free to use the drm dp helpers or use their own hooks for modeset.
I think we should look at this from drm/i915 perspective, at least at first, so
resassigning to DRM/Intel. If we come up with a way to negotiate this with
userspace, that should be a generic solution.

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