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

[Eric Anholt]

Daniel Vetter <daniel at ffwll.ch> writes:

> On Fri, Mar 31, 2017 at 05:22:09PM -0700, Eric Anholt wrote:
>> Manasi Navare <manasi.d.navare at intel.com> writes:
>> 
>> > On Fri, Mar 31, 2017 at 01:08:41PM -0700, Eric Anholt wrote:
>> >> Manasi Navare <manasi.d.navare at intel.com> writes:
>> >> 
>> >> > On Thu, Mar 30, 2017 at 05:37:55PM -0700, Eric Anholt wrote:
>> >> >> Martin Peres <martin.peres at linux.intel.com> writes:
>> >> >> 
>> >> >> > On 26/01/17 14:37, Martin Peres wrote:
>> >> >> >> 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>
>> >> >> >
>> >> >> > The relevant kernel patches have landed in drm-tip about a month ago.
>> >> >> >
>> >> >> > Eric, would you mind providing feedback on or merging this patch?
>> >> >> 
>> >> >> The later discussion has sounded like the kernel will (always) prune the
>> >> >> mode when we re-query, meaning that it doesn't make any sense to try to
>> >> >> re-set to the old mode.  Is this not the case?
>> >> >
>> >> >
>> >> > No the kernel will simply send a hotplug with link status as bad
>> >> > and then after that point its userspace driver's responsibility
>> >> > to check if link status is BAD, retry the same mode and if it fails
>> >> > then re probe.
>> >> 
>> >> So the kernel will sometimes allow the same mode to be re-set with the
>> >> same bpp?
>> >
>> > So when userspace driver re-sets the same mode, the kernel will call the
>> > mode valid function where it will see it can allow the sam mode perhaps
>> > at a lower bpp now since the link parameters are lowered.
>> > So the mode which failed at 30 bpp, might still work at 18bpp and is
>> > better going to a lower resolution.
>> 
>> The question was whether the kernel will ever allow the same mode at the
>> same bpp, since that's what this patch tries to do.
>
> Yes, this can happen. Doing a full modeset with recomputing clocks and
> everything behind userspace's back might not be something the kernel
> driver can pull of with a reasonable amount of effort, hence why we punt
> to userspace. The interface spec makes this a CAN, not WILL, to allow less
> unreasonable hw to handle these cases directly in the kernel driver. E.g.
> plain link-retraining is handled in i915.ko still.

So in that case you do need userspace to re-request the same mode at the
same bpp?
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