Re: [PATCH] drm: Document that power requirements for DP AUX transfers

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, May 05, 2022 at 08:00:20AM -0700, Doug Anderson wrote:
> Hi,
> 
> On Thu, May 5, 2022 at 7:46 AM Ville Syrjälä
> <ville.syrjala@linux.intel.com> wrote:
> >
> > On Wed, May 04, 2022 at 02:10:08PM -0400, Lyude Paul wrote:
> > > On Wed, 2022-05-04 at 09:04 -0700, Doug Anderson wrote:
> > > > Hi,
> > > >
> > > > On Wed, May 4, 2022 at 5:21 AM Ville Syrjälä
> > > > <ville.syrjala@linux.intel.com> wrote:
> > > > >
> > > > > On Tue, May 03, 2022 at 04:21:08PM -0700, Douglas Anderson wrote:
> > > > > > When doing DP AUX transfers there are two actors that need to be
> > > > > > powered in order for the DP AUX transfer to work: the DP source and
> > > > > > the DP sync. Commit bacbab58f09d ("drm: Mention the power state
> > > > > > requirement on side-channel operations") added some documentation
> > > > > > saying that the DP source is required to power itself up (if needed)
> > > > > > to do AUX transfers. However, that commit doesn't talk anything about
> > > > > > the DP sink.
> > > > > >
> > > > > > For full fledged DP the sink isn't really a problem. It's expected
> > > > > > that if an external DP monitor isn't plugged in that attempting to do
> > > > > > AUX transfers won't work. It's also expected that if a DP monitor is
> > > > > > plugged in (and thus asserting HPD) that it AUX transfers will work.
> > > > > >
> > > > > > When we're looking at eDP, however, things are less obvious. Let's add
> > > > > > some documentation about expectations. Here's what we'll say:
> > > > > >
> > > > > > 1. We don't expect the DP AUX transfer function to power on an eDP
> > > > > > panel. If an eDP panel is physically connected but powered off then it
> > > > > > makes sense for the transfer to fail.
> > > > >
> > > > > I don't agree with this. I think the panel should just get powred up
> > > > > for AUX transfers.
> > > >
> > > > That's definitely a fair thing to think about and I have at times
> > > > thought about trying to make it work that way. It always ends up
> > > > hitting a roadblock.
> >
> > How do you even probe the panel initially if you can't power it on
> > without doing some kind of full modeset/etc.?
> 
> It's not that we can't power it on without a full modeset. It' that at
> panel probe time all the DRM components haven't been hooked together
> yet, so the bridge chain isn't available yet. The panel can power
> itself on, though. This is why the documentation I added says: "if a
> panel driver is initiating a DP AUX transfer it may power itself up
> however it wants"
> 
> 
> > > > The biggest roadblock that I recall is that to make this work then
> > > > you'd have to somehow ensure that the bridge chain's pre_enable() call
> > > > was made as part of the AUX transfer, right? Since the transfer
> > > > function can be called in any context at all, we have to coordinate
> > > > this with DRM. If, for instance, DRM is mid way through powering the
> > > > panel down then we need to wait for DRM to fully finish powering down,
> > > > then we need to power the panel back up. I don't believe that we can
> > > > just force the panel to stay on if DRM is turning it off because of
> > > > panel power sequencing requirements. At least I know it would have the
> > > > potential to break "samsung-atna33xc20.c" which absolutely needs to
> > > > see the panel power off after it's been disabled.
> > > >
> > > > We also, I believe, need to handle the fact that the bridge chain may
> > > > not have even been created yet. We do AUX transfers to read the EDID
> > > > and also to setup the backlight in the probe function of panel-edp. At
> > > > that point the panel hasn't been linked into the chain. We had _long_
> > > > discussions [1] about moving these out of probe and decided that we
> > > > could move the EDID read to be later but that it was going to really
> > > > ugly to move the AUX backlight later. The backlight would end up
> > > > popping up at some point in time later (the first call to panel
> > > > prepare() or maybe get_modes()) and that seemed weird.
> > > >
> > > > [1]
> > > > https://lore.kernel.org/lkml/CAD=FV=U5-sTDLYdkeJWLAOG-0wgxR49VxtwUyUO7z2PuibLGsg@mail.gmail.com/
> > > >
> > > >
> > > > > Otherwise you can't trust that eg. the /dev/aux
> > > > > stuff is actually usable.
> > > >
> > > > Yeah, it's been on my mind to talk more about /dev/aux. I think
> > > > /dev/aux has some problems, at least with eDP. Specifically:
> > > >
> > > > 1. Even if we somehow figure out how to power the panel on as part of
> > > > the aux transfer, we actually _still_ not guaranteed to be able to
> > > > talk to it as far as I understand. My colleague reported to me that on
> > > > a system he was working with that had PSR (panel self refresh) that
> > > > when the panel was powered on but in PSR mode that it wouldn't talk
> > > > over AUX. Assuming that this is correct then I guess we'd also have to
> > > > do even more coordination with DRM to exit PSR and block future
> > > > transitions of PSR. (NOTE: it's always possible that my colleague ran
> > > > into some other bug and that panels are _supposed_ to be able to talk
> > > > in PSR. If you think this is the case, I can always try to dig more).
> > >
> > > TBH - the coordination with drm I don't think would be the difficult part, as
> > > we'd just need to add some sort of property (ideally invisible to userspace)
> > > that can be used in an atomic commit to disable PSR - similar to how we enable
> > > CRC readback from sysfs in the majority of DRM drivers. That being said
> > > though, I think we can just leave the work of solving this problem up to
> > > whoever ends up needing this to work.
> >
> > The driver should just disable/prevent PSR when doing AUX if the hardware
> > can't guarantee the PSR and AUX won't interfere with each other.
> 
> OK, fair enough. If we can solve the PSR problem that would be great.
> 
> 
> > For i915 we have no problems with powering the panel on for AUX, but
> > there is still a race with PSR vs. AUX because both use the same hardware
> > internally. I've been nagging at people to fix this for i915 but I don't
> > think it still got done :( Originally we were supposed to get a hardware
> > mutex for this but that plan got scrapped for some reason.
> 
> I haven't looked at the i915 DRM code much, but my understanding is
> that it's more of an "all in one" approach. The one driver pretty much
> handles everything itself. That means that powering the panel up isn't
> too hard. Is that right?

Yeah, we don't have too many "helpful" abstractions in the way ;)

> > > > for userspace to be mucking with /dev/aux. For DP's case I guess
> > > > /dev/aux is essentially enabling userspace drivers to do things like
> > > > update firmware on DP monitors or play with the backlight. I guess we
> > > > decided that we didn't want to add drivers in the kernel to handle
> > > > this type of stuff so we left it for userspace? For eDP, though, there
> > >
> > > The main reason DP AUX got exposed to userspace in the first place was for
> > > usecases like fwupd,
> >
> > My memory says the original reason was debugging. Or at least I had
> > no idea fwupd had started to use this until I saw some weird looking
> > DPCD addresses in some debug log.
> >
> > But I suppose it's possible there were already plans for firmware
> > updates and whatnot and it just wasn't being discussed when this was
> > being developed.
> 
> If it's just for debugging, I'd argue that leaving it as-is should be
> fine. Someone poking around with their system can find a way to make
> sure that the panel stays on.

That could require altering the state of the system quite a bit, which
may defeat the purpose. At least I would not be willing to accept such 
a limitation.

> This is similar to how if you're poking
> around with /dev/i2c it's up to you to make sure that the i2c device
> you're poking at stays powered.
> 
> -Doug

-- 
Ville Syrjälä
Intel