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

From: Manasi Navare <manasi.d.navare-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
To: Eric Anholt <eric-WhKQ6XTQaPysTnJN9+BGXg@public.gmane.org>
Cc: xorg-devel-go0+a7rfsptAfugRpC6u6w@public.gmane.org,
	dri-devel-PD4FTy7X32lNgt0PjOBp9y5qC8QIuHrW@public.gmane.org,
	Martin Peres
	<martin.peres-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
Subject: Re: [RFC PATCH xserver] modesetting: re-set the crtc's mode when link-status goes BAD
Date: Fri, 31 Mar 2017 13:17:14 -0700	[thread overview]
Message-ID: <20170331201713.GO31477@intel.com> (raw)
In-Reply-To: <87d1cxw6nq.fsf-omZaPlIz5HhaEpDpdNBo/KxOck334EZe@public.gmane.org>

On Fri, Mar 31, 2017 at 01:08:41PM -0700, Eric Anholt wrote:
> Manasi Navare <manasi.d.navare@intel.com> writes:
> 
> > On Thu, Mar 30, 2017 at 05:37:55PM -0700, Eric Anholt wrote:
> >> Martin Peres <martin.peres@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@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.

Regards
Manasi

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