Message-ID: <1448300911.4081.2.camel@redhat.com>
Date:	Mon, 23 Nov 2015 12:48:31 -0500
From:	Lyude <cpaul@...hat.com>
To:	"Deucher, Alexander" <Alexander.Deucher@....com>,
	"Koenig, Christian" <Christian.Koenig@....com>,
	Daniel Stone <daniel@...ishbar.org>
Cc:	David Airlie <airlied@...ux.ie>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jerome Glisse <jglisse@...hat.com>,
	Benjamin Tissoires <benjamin.tissoires@...hat.com>
Subject: Re: [PATCH] drm/radeon: Retry DDC probing on DVI on failure if we
 got an HPD interrupt

On Mon, 2015-11-23 at 10:43 -0500, Lyude wrote:
> On Mon, 2015-11-23 at 14:20 +0000, Deucher, Alexander wrote:
> > > -----Original Message-----
> > > From: Lyude [mailto:cpaul@...hat.com]
> > > Sent: Sunday, November 22, 2015 9:45 PM
> > > To: Koenig, Christian; Daniel Stone
> > > Cc: Deucher, Alexander; David Airlie; dri-devel; Linux Kernel Mailing
> > > List;
> > > Jerome Glisse; Benjamin Tissoires
> > > Subject: Re: [PATCH] drm/radeon: Retry DDC probing on DVI on failure if we
> > > got an HPD interrupt
> > > 
> > > 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@amd
> > > > > .com> wrote:
> > > > > > On 20.11.2015 16:52, cpaul@...hat.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@...hat.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.
> > 
> > FWIW, there are registers to adjust how long the hpd needs to be asserted
> > before the hpd connection and short pulse interrupts are triggered.  See
> > DC_HPDx_CONTROL.  Maybe adjusting them would help.  We currently just write
> > the default value, but it might be better to RMW the value in case there is
> > a
> > special golden value set by the vbios at init time.
> > 
> This sounds like a much better idea. I'm going to try to come up with a patch
> that adjusts these by default for DVI ports.

Ignore this; we can't get a long enough delay using those registers to fix the
issue so we'll still have to handle this in software.
> > Alex
> > 
> > > > 
> > > > Regards,
> > > > Christian.
> > > > 
> > > > > 
> > > > > Cheers,
> > > > > Daniel
> > > > 
> > > --
> > > Cheers,
> > > 	Lyude
> > 
-- 
Cheers,
	Lyude

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists