Re: [PATCH igt] lib/kms: Force a full reprobe if we find a bad link

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Martin Peres <martin.peres@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH igt] lib/kms: Force a full reprobe if we find a bad link
Date: Wed, 31 May 2017 14:55:14 +0100	[thread overview]
Message-ID: <20170531135514.GS23936@nuc-i3427.alporthouse.com> (raw)
In-Reply-To: <528f43bb-e448-e02d-21c7-59b9f591132b@linux.intel.com>

On Wed, May 31, 2017 at 04:44:41PM +0300, Martin Peres wrote:
> On 31/05/17 15:42, Chris Wilson wrote:
> >On Wed, May 31, 2017 at 01:40:00PM +0300, Martin Peres wrote:
> >>On 26/05/17 14:48, Chris Wilson wrote:
> >>>If we do a shallow probe of the connector and it reports the link failed
> >>>previous (link-status != GOOD), force a full probe of the connector to
> >>>give the kernel a chance to validate the mode list.
> >>
> >>Sounds good, but will this make the tests SKIP if no modes are available?
> >
> >I'm actually not sure what will happen if the mode is removed. I think
> >the tests are just using the first mode in the list? At the moment I
> >hope just to stop turning a single failure into many, it is still a bug
> >that the link training failed and was not recovered. Alternatively, we
> >can ask why isn't the kernel taking the corrective action when presented
> >with a new setcrtc?
> 
> No, this is not a kernel bug, it is a failure that the userspace has
> to handle because the kernel can't do shit about this.

Have you demonstrated that the kernel is beyond reproach when it failed
the link training? Nothing changed in the connection and it works most
of the time, so why did the kernel accept the failure. Even if we
temporarily force a change of modes that is poor UX that I see no reason
why it should not have been prevented in the first place.

> >I'm not sure what the correct approach here should be, just what is the
> >contract the kernel is expecting of userspace? Should that contract
> >apply to new clients unaware of the earlier error?
> 
> Right, IGT assumes that if a mode is already set, it can be set
> again. However, this assumption has been broken when the link-status
> patches landed.
> 
> On a hotplug event, IGT should do a full reprobe, select one mode
> from the list and use it. If no modes can be set and the test is
> trying to set one, then the test should just SKIP.

There is no hotplug event when a new client starts so how is igt meant
to even know that it was supposed to pick up the pieces for the kernel.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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