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

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Date:	Sat, 21 Nov 2015 16:22:28 +0100
From:	Christian König <christian.koenig@....com>
To:	Daniel Stone <daniel@...ishbar.org>
CC:	<cpaul@...hat.com>, Alex Deucher <alexander.deucher@....com>,
	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 21.11.2015 15:49, Daniel Stone wrote:
> Hi,
>
> On 21 November 2015 at 14:22, Christian König <christian.koenig@....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.
>>
>> Retrying on the other hand doesn't necessarily improve the situation cause
>> the delay introduced by this might not be enough.
>>
>> 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?

Regards,
Christian.

>
> Cheers,
> Daniel

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