Date:   Fri, 6 May 2022 01:15:34 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Doug Anderson <dianders@...omium.org>
Cc:     Ville Syrjälä <ville.syrjala@...ux.intel.com>,
        Lyude Paul <lyude@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Jani Nikula <jani.nikula@...el.com>,
        Maxime Ripard <maxime@...no.tech>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Robert Foss <robert.foss@...aro.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH] drm: Document that power requirements for DP AUX
 transfers

On 06/05/2022 00:24, Doug Anderson wrote:
> Hi,
> 
> On Thu, May 5, 2022 at 1:56 PM Dmitry Baryshkov
> <dmitry.baryshkov@...aro.org> wrote:
>>
>> On Thu, 5 May 2022 at 23:21, Doug Anderson <dianders@...omium.org> wrote:
>>>
>>> Hi,
>>>
>>> On Thu, May 5, 2022 at 1:10 PM Dmitry Baryshkov
>>> <dmitry.baryshkov@...aro.org> wrote:
>>>>
>>>> On Thu, 5 May 2022 at 18:53, Doug Anderson <dianders@...omium.org> wrote:
>>>>>
>>>>> Hi,
>>>>>
>>>>> On Thu, May 5, 2022 at 8:29 AM Ville Syrjälä
>>>>> <ville.syrjala@...ux.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 Syrjälä
>>>>>>> <ville.syrjala@...ux.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@...ux.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?
>>>>>
>>>>> 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.
>>>>>
>>>>>
>>>>>>   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 my 2c counts, I support landing this patch. It clearly documents
>>>> current behaviour and expectations.
>>>>
>>>> If that helps,
>>>> Acked-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
>>>>
>>>> As for the /dev/aux, question, I think we can make the following plan work:
>>>> - Document that eDP panel power up can be handled by using the
>>>> pm_runtime API (which is the case for both panel-edp and atna33xc20)).
>>>> I think this is a sensible requirement anyway. And both panels show
>>>> how to handle different poweron/poweroff timings.
>>>> - Make drm_dp_aux_dev_get_by_minor() pm_runtime_get() the attached panel.
>>>
>>> This matches what you suggested previously, but I still think it has a
>>> potential problem as I talked about in the my previous (very long)
>>> reply [1]. The relevant part was:
>>>
>>>> Now, despite the fact that the generic eDP panel code doesn't follow
>>>> the "strict"ness I just described, the _other_ DP panel I worked on
>>>> recently (samsung-atna33xc20) does. In testing we found that this
>>>> panel would sometimes (like 1 in 20 times?) crash if you ever stopped
>>>> outputting data to the display and then started again. You absolutely
>>>> needed to fully power cycle the display each time. I tried to document
>>>> this to the best of my ability in atana33xc20_unprepare(). There's
>>>> also a WARN_ON() in atana33xc20_enable() trying to detect if someone
>>>> is doing something the panel driver doesn't expect.
>>>
>>> Specifically, I think you could get in trouble if you did:
>>>
>>> a) drm wants to power down the panel.
>>>
>>> b) drm calls the panel's disable() function
>>>
>>> c) we start an aux transfer and grab a runtime pm reference
>>>
>>> d) drm calls the panel's unprepare() function => atana33xc20_unprepare()
>>>
>>> e) atana33xc20_unprepare()'s pm_runtime_put_sync_suspend() _won't_
>>> power off the panel (we still have the reference from step c), even
>>> though it needs to.
>>>
>>> f) we'll finish an aux transfer and, presumably, call
>>> pm_runtime_put_autosuspend()
>>>
>>> g) drm wants to power the panel back up
>>>
>>> h) drm calls the panel's prepare() function, but power wasn't properly cycled
>>
>> Why? We'd need to extend the prepare() function with the flag
>> data_was_on, which is set in the enable() and cleared in the suspend
>> path. If we see this flag in the prepare() callback, we should
>> forcibly power cycle the panel by toggling the regulator. Yes, this
>> will cause additional wait.
>>
>> Another option might be to toggle the autosuspend support. Let the
>> disable() call pm_runtime_dont_autosuspend() (which would turn all
>> autosuspend calls into plain pm_runtime_put()) and allow it again in
>> the resume(). I'm not 100% sure that this will work, but it looks like
>> it should.
> 
> It turns out, though, that we _want_ autosuspend sometimes, even when
> the panel is disabled. Specifically, if the panel is disabled and then
> atana33xc20_get_modes() gets called then we _don't_ want to fully
> power the panel off. It's expected that there will be a future call to
> prepare() soon after the get_modes() and we don't want a full power
> cycle (500 ms) there. This mechanism is also fully allowed by the eDP
> spec. The only time we _need_ a full power cycle is after disable().

Yes. That's why I proposed to put pm_runtime_dont_autosuspend() call to 
the disable(). So that after get_modes() there will be no power cycle, 
but only after disable() call.

> 
> In any case, we can come up with complex ways to solve this, but I'm
> still just not convinced that it's worth it. There's no valid use case
> other than debugging. IMO if we're poking around and want to read DP
> registers while the panel is on then it works just fine. A user doing
> this can ensure that the panel is on while poking. Certainly I've done
> that and it wasn't a big imposition.
> 
> If someone wants to submit patches to attempt this then I'm happy to
> test them, but I feel like it's adding complexity for very little
> value. The way it works now is simple / understandable and mathes my
> intuition from other busses, like i2c. The bus is just responsible for
> powering itself, not the devices on the bus. It has also long been
> documented since commit 83127f67e450 ("drm/panel: Flesh out
> kerneldoc") in 2016 that the way to turn on a panel for communication
> over the command bus is via drm_panel_prepare(). I don't think we need
> to change this.

Yes.


>> The second option leaves a possible window for the panel issues if the
>> userspace AUX transfer is ongoing while the panel is being unprepared
>> and then prepared again. In this case it will never be power cycled.
>> However after some thought I think this is correct. You wouldn't like
>> to power cycle a panel while you are e.g. updating the firmware.
> 
> As per my earlier responses, nothing we are doing here solves the
> firmware update anyway. Even if we automatically power the panel for
> the duration of a single aux transfer, nothing prevents the panel from
> turning off between transfers. There's no API to "keep the power on
> until I say stop". You certainly wouldn't want a panel to turn off
> midway through a firmware update. IMO if we want to use this for
> firmware update, we either need an special way to force the panel on
> (in which case, we don't need to worry about it in the aux transfer
> function) or (better IMO) we need to manage the firmware update in the
> panel driver and prevent some type of sysfs interface to provide the
> new firmware and kick off the update. Presumably having this managed
> by the panel driver would be best because the panel driver could know
> to, for instance, re-read the EDID after the firmware update took
> place.

That's why I suggested pm_runtime_get()'ting the panel from 
drm_dp_aux_dev_get_by_minor(). This way the whole userspace session will 
be protected.

But as you said, this becomes more and more complex with little added value.

-- 
With best wishes
Dmitry

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