Re: [PATCH 1/2] drm/edid: Add helper to detect whether EDID changed

[Paul Kocialkowski]

On Tue, 2017-07-25 at 10:16 +0200, Daniel Vetter wrote:
> On Tue, Jul 25, 2017 at 10:58:55AM +0300, Paul Kocialkowski wrote:
> > On Tue, 2017-07-25 at 09:34 +0200, Daniel Vetter wrote:
> > > On Tue, Jul 25, 2017 at 9:25 AM, Paul Kocialkowski
> > > <paul.kocialkowski@xxxxxxxxxxxxxxx> wrote:
> > > > On Tue, 2017-07-25 at 08:53 +0200, Daniel Vetter wrote:
> > > > > On Mon, Jul 24, 2017 at 05:54:46PM +0300, Paul Kocialkowski
> > > > > wrote:
> > > > > > This adds a common drm helper to detect whether the EDID
> > > > > > changed
> > > > > > from
> > > > > > the last known cached one. This is useful help detect that a
> > > > > > monitor
> > > > > > was
> > > > > > changed during a suspend/resume cycle.
> > > > > > 
> > > > > > When that happens (a monitor is replaced by another one
> > > > > > during
> > > > > > suspend),
> > > > > > no hotplug event will be triggered so the change will not be
> > > > > > caught
> > > > > > at
> > > > > > resume time. Detecting that the EDID changed allows
> > > > > > detecting
> > > > > > it.
> > > > > > 
> > > > > > Signed-off-by: Paul Kocialkowski <paul.kocialkowski@xxxxxxxx
> > > > > > tel.
> > > > > > com>
> > > > > 
> > > > > I can't find the older mails I've typed about this, but the
> > > > > plan
> > > > > we've
> > > > > discussed a while back was:
> > > > > - Add a generational counter to each connector, maybe even
> > > > > expose
> > > > > it
> > > > > to
> > > > >   userspace.
> > > > > 
> > > > > - Increment that counter every time something changed, e.g.
> > > > >   connector->status in the propbe helpers, or when attaching a
> > > > > new
> > > > > edid
> > > > >   with the set_edid helper.
> > > > > 
> > > > > - Tada, no changes needed to drivers, and easily extensible to
> > > > > other
> > > > >   things than edid!
> > > > 
> > > > I don't see how it solves the problem here though. After a
> > > > suspend/resume cycle, there is simply no indication that
> > > > anything
> > > > has
> > > > changed when a monitor was replaced by another one, so I don't
> > > > see
> > > > how
> > > > adding a counter in the mix would help.
> > > > 
> > > > Could you provide more details about the reasoning? I feel like
> > > > I'm
> > > > missing something here.
> > > 
> > > Your bug doesn't just exist over s/r, it's just much easier to
> > > observe
> > > in s/r since users can take however long they want to with
> > > plugging in
> > > a different monitor. But the same issue exists e.g. when we go
> > > from
> > > hpd to polling because too much noise on the line.
> > > 
> > > Wrt the suspend/resume issue: What we need to do on resume is do a
> > > full reprobe of all outputs, in an async worker. Telling userspace
> > > to
> > > do this by sending an uevent was the cheapest way, but it'd be
> > > better
> > > if the kernel could do that asynchronously and inform userspace
> > > about
> > > the exact changes. And there's more to reprobe than just the edid,
> > > and
> > > we don't want to re-invent a separate reprobe path just for resume
> > > like you start in your patch series. So yeah my plan was missing:
> > > 
> > > - force a full async reprobe after resume (maybe we could reuse
> > > the
> > > poll worker for that as a one-shot).
> > 
> > First off, I definitely agree we need a way to tell userspace
> > exactly
> > what has happened. I wanted to start a discussion about that in i-g-
> > t
> > patch "Unrelated hotplug uevent masking out actual test result" but
> > it
> > didn't get much traction. For testing purposes, it is unacceptable
> > that
> > userspace only gets notified that "something happened".
> > 
> > Still, as far as I know, userspace is expected to ask for a full
> > reprobe
> > when something has changed, and that is apparently part of the DRM
> > spec,
> > so we can't expect that it could query for an update on "only the
> > things
> > that changed".
> 
> We can update that spec in a backwards compatible way. E.g. we can ask
> for
> the current properties without forcing a reprobe (won't even call down
> into the driver), and userspace could use that to check which
> connector
> has an incremented epoche counter since the last time it sampled
> things.
> Then it can reprobe just that one.
> 
> Old userspace wouldn't know about this, and would keep working as-is.

So the level of detail you're aiming at providing userspace is
"connector foo changed" then? I agree it is better than the current
"some connector(s) changed", but what I'd like to see for proper testing
is a way to find out "bar for connector foo changed".

> > However, one way to mitigate this is to make sure that the driver
> > knows
> > what changed and only updates these things when a full reprobe is
> > requested. Is this the approach that you have in mind?
> > 
> > The methodology behind my series follows what is currently done:
> > detect
> > change in whatever way necessary, inform userspace and let it
> > trigger
> > full reprobe. If I'm understanding correctly, what you're suggesting
> > is
> > instead to reprobe what is needed on the kernel side when an
> > associated
> > change occurs instead of having userspace trigger it, and then let
> > userspace aware that something changed and return the "cached"
> > updated
> > status when userspace asks for the subsequent reprobe. Is that
> > correct?
> 
> There's two things: the uapi discussion and the internal
> implementation,
> imo their separate (but somewhat connected) topics.
> 
> - For the internal implementation of detecting edid changes I don't
> like
>   your approach of rolling a completely new detect path just for
> resume.
>   I think we can very well integrate that into the existing probe code
>   using the approach I've laid out.
> 
> - There's more than just edid (e.g. hdcp status, various stuff that's
>   handled in dp aux for DP sinks), and I think a general mechanism for
>   tracking that something changed will be useful for the internal
>   implementation. The other plan would be that we have to wire a bool
>   changed through the entire probe stack, and make sure it's handled
>   correctly everywhere, which is a) a lot more work b) more fragile.
> Doing
>   a connector->status_epoch++ everywhere we detect a change is _much_
>   simpler.

So to summarize, the following would happen: an async worker would
detect whether something changed, then increase the counter for that
connector and notify userspace, which would trigger full reprobe of that
connector only. Legacy userspace would just trigger full reprobe for all
connectors.

I am still under the impression that you'd like the full reprobe to be
done on the kernel's async worker, to detect that e.g. EDID changed. But
then userspace is going to fully reprobe again, so it will be
duplicated. Unless the kernel also keeps a reference of the last time
the counter was read from userspace, to determine when to skip full
reprobe when it is asked from userspace? That feels pretty similar to
having a bool indicating change.

My approach here was to look specifically for the thing that can change
in the async worker (only EDID with this change, but it could be
extended for the other things you mentioned) as to reduce the
duplication as much as possible.

> - For the uapi change: We already support returning the cached stuff,
> the
>   only bit that's missing is the epoch counter to let userspace know
> where
>   it might need to do a full reprobe. Or maybe we'll just spec that a
> full
>   reprobe isn't necessary after a hpd event (but that's unlikely to
> work
>   out given how many bugs we'd need to fix first).

Okay, thanks for the additional explanation. I think I'm getting a
better grasp on your idea.

-- 
Paul Kocialkowski <paul.kocialkowski@xxxxxxxxxxxxxxx>
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