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

[Jani Nikula]

On Tue, 31 Jan 2017, Eric Anholt <eric at anholt.net> wrote:
> 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?

Disclaimer: I don't know anything about the internals of the modesetting
driver.

Perhaps we can identify the two cases now, but I'd put this more
generally: if the link status has gone bad, it's an indicator to
userspace that the circumstances may have changed, and userspace should
query the kernel for the list of available modes again. It should no
longer trust information obtained prior to getting the bad link status,
including the current mode.

But specifically, I think you're right, and AFAICT asking for the list
of modes again is the only way for the userspace to distinguish between
the two cases. I don't think there's a shortcut for deciding the current
mode is still valid.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Technology Center