[RFC PATCH xserver] modesetting: re-set the crtc's mode when link-status goes BAD

[Eric Anholt]

Martin Peres <martin.peres at linux.intel.com> writes:

> Despite all the careful planing of the kernel, a link may become
> insufficient to handle the currently-set mode. At this point, the
> kernel should mark this particular configuration as being broken
> and potentially prune the mode before setting the offending connector's
> link-status to BAD and send the userspace a hotplug event. This may
> happen right after a modeset or later on.
>
> When available, we should use the link-status information to reset
> the wanted mode.
>
> Signed-off-by: Martin Peres <martin.peres at linux.intel.com>

If I understand this right, there are two failure modes being handled:

1) A mode that won't actually work because the link isn't good enough.

2) A mode that should work, but link parameters were too optimistic and
if we just ask the kernel to set the mode again it'll use more
conservative parameters that work.

This patch seems good for 2).  For 1), the drmmode_set_mode_major is
going to set our old mode back.  Won't the modeset then fail to link
train again, and bring us back into this loop?  The only escape that I
see would be some other userspace responding to the advertised mode list
changing, and then asking X to modeset to something new.

To avoid that failure busy loop, should we re-fetching modes at this
point, and only re-setting if our mode still exists?
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