Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 5 May 2022 22:19:22 +0300
From:   Ville =?iso-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>
To:     Doug Anderson <dianders@chromium.org>
Cc:     Lyude Paul <lyude@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Hsin-Yi Wang <hsinyi@chromium.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Stephen Boyd <swboyd@chromium.org>,
        Jani Nikula <jani.nikula@intel.com>,
        Maxime Ripard <maxime@cerno.tech>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        freedreno <freedreno@lists.freedesktop.org>,
        Robert Foss <robert.foss@linaro.org>,
        Laurent Pinchart <Laurent.pinchart@ideasonboard.com>
Subject: Re: [PATCH] drm: Document that power requirements for DP AUX
 transfers
Message-ID: <YnQjOuSh3TV+Zbgk@intel.com>
References: <20220503162033.1.Ia8651894026707e4fa61267da944ff739610d180@changeid>
 <YnJv3B/85hTz54SC@intel.com>
 <CAD=FV=WndmKuEB0=OVQP9YuJaSmD0uxkNs5LE0wWsFj7gBvhBA@mail.gmail.com>
 <1c6c9fde6e85f09cc89ea8dc6e8716fef58f3ee1.camel@redhat.com>
 <YnPjO4kbjezQl5Da@intel.com>
 <CAD=FV=XbZEagm5qR207mcVm1Ry=bGeuRAqTYx3SBoZfyo6fSkg@mail.gmail.com>
 <YnPoYsnx7IeBfJ5D@intel.com>
 <CAD=FV=WxxEGM4cLBHGMeRBFDAXGJJF105kLZ588JSFJRg8PM8A@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CAD=FV=WxxEGM4cLBHGMeRBFDAXGJJF105kLZ588JSFJRg8PM8A@mail.gmail.com>
Precedence: bulk

On Thu, May 05, 2022 at 08:53:12AM -0700, Doug Anderson wrote:
> Hi,
> 
> On Thu, May 5, 2022 at 8:29 AM Ville Syrj?l?
> <ville.syrjala@linux.intel.com> wrote:
> >
> > On Thu, May 05, 2022 at 08:00:20AM -0700, Doug Anderson wrote:
> > > Hi,
> > >
> > > On Thu, May 5, 2022 at 7:46 AM Ville Syrj?l?
> > > <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 Syrj?l?
> > > > > > <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.
> 
> It does? In my experience you just need to make sure that the panel is
> turned on. ...or are you saying that you'd use this for debugging a
> case where the system isn't probing properly?

I don't want to have to eg. try to set a mode on the panel to get it
to reply to AUX/DDC. I want to be able to talk to it in any situation.
I don't need to jump through any extra hoops to talk to external DP
panels, and don't really see why eDP should be any different.

> If things are truly in bad shape, at least on boards using device tree
> it's easy to tweak the device tree to force a regulator to stay on. I
> suppose we could also add a "debugfs" entry for the panel that also
> forces it to be powered on.

Not really sure how adding a separate knob for it would be
somehow easier than just turning on the power from the
aux transfer hook.

> >  At least I would not be willing to accept such
> > a limitation.
> 
> Hmm, so where does that leave us? Are you against landing this patch?
> I've done a lot of cleanups recently and I just don't think I have the
> time to rework all the AUX transfer functions and figure out how to
> power the panel. It also seems like a lot of added complexity for a
> debug path.

If people don't feel like fixing this then I have no real
objection to documenting the fact that *some* drivers can't
power the panel on for AUX transfers for whatever random
reason. But I disagree with claims that it is the only
expected/desired behaviour.

-- 
Ville Syrj?l?
Intel