[PATCH] drm/radeon: Retry DDC probing on DVI on failure if we got an HPD interrupt

[Lyude]

On Sat, 2015-11-21 at 16:22 +0100, Christian König wrote:
> On 21.11.2015 15:49, Daniel Stone wrote:
> > Hi,
> > 
> > On 21 November 2015 at 14:22, Christian König <christian.koenig at amd
> > .com> wrote:
> > > On 20.11.2015 16:52, cpaul at redhat.com wrote:
> > > > This is somewhat rare on most cards (depending on what angle
> > > > you plug
> > > > the DVI connector in), but on some cards it happens constantly.
> > > > The
> > > > Radeon R5 on the machine used for testing this patch for
> > > > instance, runs
> > > > into this issue just about every time I try to hotplug a DVI
> > > > monitor and
> > > > as a result hotplugging almost never works.
> > > > 
> > > > Rescheduling the hotplug work for a second when we run into an
> > > > HPD
> > > > signal with a failing DDC probe usually gives enough time for
> > > > the rest
> > > > of the connector's pins to make contact, and fixes this issue.
> > > > 
> > > > Signed-off-by: Stephen Chandler Paul <cpaul at redhat.com>
> > > 
> > > Yeah, that's something I always wondered a about bit as well.
> > > 
> > > Debouncing is something very common done in electronics, but as
> > > far as I
> > > know the HPD pins don't necessary have an RC circuit so we might
> > > need to
> > > handle this case in software here.
> > > 
> > > A delay of something between 10-30ms between the last HPD
> > > interrupt and
> > > further processing of the signal doesn't sounds like such a bad
> > > idea.
Unfortunately the delay needed to make hotplugging work on the system
mentioned in the commit log can actually be over 700ms.
> > > 
> > > Retrying on the other hand doesn't necessarily improve the
> > > situation cause
> > > the delay introduced by this might not be enough.
Yeah, but I would think it would make sense to retry here so long as we
back off after a certain time. This would also have the benefit of
skipping this delay on systems that don't need it.
> > > 
> > > So I would rather vote for a fixed delay between an HPD interrupt
> > > and
> > > actually starting to process anything.
> > Yes-ish. Debouncing is useful, and ignoring buggy devices (e.g.
> > those
> > on marginal power) which send you HPD storms as well. But DP relies
> > on
> > 'short HPD' pulses which can be as brief as 2ms. So attempting to
> > totally debounce all HPD won't work.
> Well the discussion so far was about HPD on DVI only.
> 
> I'm not so deep into DP, but why should it uses HPD pulses of less
> than 2ms?
This is part of the DP spec iirc. This being said though, the issue
here with the HPD signal coming before the connector is ready only
happens on DVI. I haven't ever run into this issue with any HDMI cables
or DP cables, so I'm against imposing this on all connectors.

One of the solutions I've been thinking about with this: In
radeon_dvi_detect(), if we get a real hotplug signal retry the DDC
probe until at least a second has passed, after which we back off and
assume the port is disconnected.
> 
> Regards,
> Christian.
> 
> > 
> > Cheers,
> > Daniel
> 
-- 
Cheers,
	Lyude