Re: [igt-dev] Improving DRM connector hotplug uevents

From: "Ser, Simon" <simon.ser@intel.com>
To: "daniel.vetter@ffwll.ch" <daniel.vetter@ffwll.ch>,
	"paul.kocialkowski@bootlin.com" <paul.kocialkowski@bootlin.com>
Cc: "airlied@linux.ie" <airlied@linux.ie>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"igt-dev@lists.freedesktop.org" <igt-dev@lists.freedesktop.org>,
	"thomas.petazzoni@bootlin.com" <thomas.petazzoni@bootlin.com>,
	"sean@poorly.run" <sean@poorly.run>
Subject: Re: [igt-dev] Improving DRM connector hotplug uevents
Date: Fri, 10 May 2019 14:39:05 +0000	[thread overview]
Message-ID: <2be4c7cb5b26c5bd802deb8d475f7b1f878b5a82.camel@intel.com> (raw)
In-Reply-To: <CAKMK7uHAWEptEHAbAAS3FX2UHnyc5Dmix2J9fDU8uYVLYOz4Tw@mail.gmail.com>

On Fri, 2019-05-10 at 09:56 +0200, Daniel Vetter wrote:
> On Fri, May 10, 2019 at 9:42 AM Paul Kocialkowski
> <paul.kocialkowski@bootlin.com> wrote:
> > Hi,
> > 
> > So this thread has drifted off from IGT to a general DRM improvement,
> > so renaming the thread and adding some CCs.
> > 
> > It's about avoiding full reprobes (which may include slow EDID reads,
> > etc) when a connector change was detected, by providing information
> > about the connector and the new state to userspace. This way, userspace
> > can reprobe the changed connector alone (or not reprobe at all in case
> > of a disconnect).
> > 
> > It feels like there is growing temptation to have per-driver hacks to
> > avoid full reprobes when the driver knows that nothing changed, but I
> > think it makes more sense to fixup the uevent we send to make sure that
> > userspace knows what to probe exactly.
> > 
> > On Fri, 2019-05-10 at 06:55 +0000, Ser, Simon wrote:
> > > On Thu, 2019-05-09 at 11:56 -0400, Lyude Paul wrote:
> > > > I'm in support of this as well, we really could use better hotplug events.
> > > > Feel free to cc patches to me for review :)
> > > 
> > > Any idea(s) how the API would look like?
> > 
> > From a quick chat on IRC yesterday, it seems that just adding entries
> > to the uevent would do and would be backwards-compatible (well,
> > provided that the userspace uevent parsers will not fail if something
> > extra to "HOTPLUG=1" is provided).
> > 
> > Currently, the notification is sent globally at each round of detected
> > connector change (which may concern multiple connectors). So I think we
> > should just list the connectors that changed in the uevent and their
> > new state. So my proposal here is something like:
> > 
> > HOTPLUG=1
> > CONNECTOR_ID=47
> > STATUS=Connected
> > CONNECTOR_ID=48
> > STATUS=Disconnected
> > 
> > Where each STATUS entry would refer to the previous CONNECTOR_ID entry.
> > 
> > What do you think?
> 
> We have the better uevent already, all we need is wiring up, lots of
> code, and userspace for it. Ok, not quite, patch is still in flight:
> 
> https://patchwork.kernel.org/patch/10906677/

Excellent.

I can help with a userspace implementation if needed.
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