Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753598AbbKWPnG (ORCPT ); Mon, 23 Nov 2015 10:43:06 -0500 Received: from mx1.redhat.com ([209.132.183.28]:44444 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753266AbbKWPnE (ORCPT ); Mon, 23 Nov 2015 10:43:04 -0500 Message-ID: <1448293381.4081.1.camel@redhat.com> Subject: Re: [PATCH] drm/radeon: Retry DDC probing on DVI on failure if we got an HPD interrupt From: Lyude To: "Deucher, Alexander" , "Koenig, Christian" , Daniel Stone Cc: David Airlie , dri-devel , Linux Kernel Mailing List , Jerome Glisse , Benjamin Tissoires Date: Mon, 23 Nov 2015 10:43:01 -0500 In-Reply-To: References: <1448034740-30193-1-git-send-email-cpaul@redhat.com> <56507E0F.7030009@amd.com> <56508C34.1030909@amd.com> <1448246674.26668.9.camel@redhat.com> Organization: Red Hat Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4572 Lines: 111 On Mon, 2015-11-23 at 14:20 +0000, Deucher, Alexander wrote: > > -----Original Message----- > > From: Lyude [mailto:cpaul@redhat.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 > > > .com> wrote: > > > > > On 20.11.2015 16:52, cpaul@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 > > > > > > > > > > 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. > 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@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/