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Message-ID: <ddb8d8fa-89dc-268b-0505-9ee7df8c272e@linaro.org>
Date:   Sat, 16 Apr 2022 03:54:35 +0300
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Subject: Re: [RFC PATCH 1/6] drm/dp: Helpers to make it easier for drivers to
 use DP AUX bus properly
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To:     Doug Anderson <dianders@chromium.org>
Cc:     dri-devel <dri-devel@lists.freedesktop.org>,
        Robert Foss <robert.foss@linaro.org>,
        Hsin-Yi Wang <hsinyi@chromium.org>,
        Abhinav Kumar <quic_abhinavk@quicinc.com>,
        Sankeerth Billakanti <quic_sbillaka@quicinc.com>,
        Philip Chen <philipchen@chromium.org>,
        Stephen Boyd <swboyd@chromium.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@linux.ie>,
        Linus Walleij <linus.walleij@linaro.org>,
        Lyude Paul <lyude@redhat.com>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        LKML <linux-kernel@vger.kernel.org>
References: <20220409023628.2104952-1-dianders@chromium.org>
 <20220408193536.RFC.1.I4182ae27e00792842cb86f1433990a0ef9c0a073@changeid>
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 <CAD=FV=UmXzPyVOa-Y0gpY0qcukqW3ge5DBPx6ak88ydEqTsBiQ@mail.gmail.com>
From:   Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
In-Reply-To: <CAD=FV=UmXzPyVOa-Y0gpY0qcukqW3ge5DBPx6ak88ydEqTsBiQ@mail.gmail.com>
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On 16/04/2022 03:09, Doug Anderson wrote:
> Hi,
> 
> On Fri, Apr 15, 2022 at 3:45 PM Dmitry Baryshkov
> <dmitry.baryshkov@linaro.org> wrote:
>>
>> On Sat, 16 Apr 2022 at 00:13, Doug Anderson <dianders@chromium.org> wrote:
>>>
>>> Hi,
>>>
>>> On Thu, Apr 14, 2022 at 5:47 PM Dmitry Baryshkov
>>> <dmitry.baryshkov@linaro.org> wrote:
>>>>
>>>> On 09/04/2022 05:36, Douglas Anderson wrote:
>>>>> As talked about in the kerneldoc for "struct dp_aux_ep_client" in this
>>>>> patch and also in the past in commit a1e3667a9835 ("drm/bridge:
>>>>> ti-sn65dsi86: Promote the AUX channel to its own sub-dev"), to use the
>>>>> DP AUX bus properly we really need two "struct device"s. One "struct
>>>>> device" is in charge of providing the DP AUX bus and the other is
>>>>> where we'll try to get a reference to the newly probed endpoint
>>>>> devices.
>>>>>
>>>>> In ti-sn65dsi86 this wasn't too difficult to accomplish. That driver
>>>>> is already broken up into several "struct devices" anyway because it
>>>>> also provides a PWM and some GPIOs. Adding one more wasn't that
>>>>> difficult / ugly.
>>>>>
>>>>> When I tried to do the same solution in parade-ps8640, it felt like I
>>>>> was copying too much boilerplate code. I made the realization that I
>>>>> didn't _really_ need a separate "driver" for each person that wanted
>>>>> to do the same thing. By putting all the "driver" related code in a
>>>>> common place then we could save a bit of hassle. This change
>>>>> effectively adds a new "ep_client" driver that can be used by
>>>>> anyone. The devices instantiated by this driver will just call through
>>>>> to the probe/remove/shutdown calls provided.
>>>>>
>>>>> At the moment, the "ep_client" driver is backed by the Linux auxiliary
>>>>> bus (unfortunate naming--this has nothing to do with DP AUX). I didn't
>>>>> want to expose this to clients, though, so as far as clients are
>>>>> concerned they get a vanilla "struct device".
>>>>
>>>> I have been thinking about your approach for quite some time. I think
>>>> that enforcing a use of auxilliary device is an overkill. What do we
>>>> really need is the the set callbacks in the bus struct or a notifier. We
>>>> have to notify the aux_bus controller side that the client has been
>>>> probed successfully or that the client is going to be removed.
>>>
>>> It seems like these new callbacks would be nearly the same as the
>>> probe/remove callbacks in my proposal except:
>>>
>>> * They rely on there being exactly 1 AUX device, or we make it a rule
>>> that we wait for all AUX devices to probe (?)
>>
>> Is the backlight a separate device on an AUX bus? Judging from
>> drm_panel_dp_aux_backlight(), it isn't. I assumed that aux bus is just
>> a point-to-point bus, so there is always a single client.
> 
> Define "device". ;-)

"a device on the AUX bus" = the device, which lists dp_aux_bus_type as 
dev->bus_type.

> 
> It's a seperate "struct device" from a Linux point of view since it's
> a backlight class device. Certainly it's highly correlated to the
> display, but one can conceptually think of them as different devices,
> sorta. ;-)
> 
> I actually dug a tiny bit more into the whole "touchscreen over aux".
> I guess DP 1.2 has a standard of "USB over DP AUX". No idea how that
> would be modeled, of course.

Ugh. Do you have any details of the standard itself? Like how does it 
looks like from the host point of view. And if the AUX is required to be 
powered for this USB bus to work?

In other words: if we were to model it at this moment, would it be the 
child of the panel device (like backlight) or a separate device sitting 
on the same AUX bus?

> 
> I guess the summary is that I'm OK w/ changing it to assume one device
> for now, but I'm still not sure it's compelling to move to normal
> callbacks. The API for callbacks is pretty much the same as the one I
> proposed and IMO leaving it the way it is (with an extra struct
> device) doesn't really add much complexity and has a few (small) nice
> benefits.

I think Stephen didn't like too many similarities between 
dp_aux_ep_client and dp_aux_ep_device. And I'd second him here.


>>> * We need to come up with a system for detecting when everything
>>> probes or is going to be removed, though that's probably not too hard.
>>> I guess the DP AUX bus could just replace the panel's probe function
>>> with its own and essentially "tail patch" it. I guess it could "head
>>> patch" the remove call? ...or is there some better way you were
>>> thinking of knowing when all our children probed?
>>>
>>> * The callback on the aux bus controller side would not be able to
>>> DEFER. In other words trying to acquire a reference to the panel can
>>> always be the last thing we do so we know there can be no reasons to
>>> defer after. This should be doable, but at least in the ps8640 case it
>>> will require changing the code a bit. I notice that today it actually
>>> tries to get the panel side _before_ it gets the MIPI side and it
>>> potentially can return -EPROBE_DEFER if it can't find the MIPI side. I
>>> guess I have a niggling feeling that we'll find some reason in the
>>> future that we can't be last, but we can probably ignore that. ;-)
>>>
>>> I can switch this all to normal callbacks if that's what everyone
>>> wants, but it doesn't feel significantly cleaner to me and does seem
>>> to have some (small) downsides.
>>>
>>>
>>>> And this
>>>> approach would make driver's life easier, since e.g. the bus code can
>>>> pm_get the EP device before calling callbacks/notifiers and
>>>> pm_put_autosuspend it afterwards.
>>>
>>> Not sure about doing the pm calls on behalf of the EP device. What's
>>> the goal there?
>>
>> I think any driver can pm_runtime_get another device. The goal is to
>> let the 'post_probe' callback to power up the panel, read the EDID,
>> etc.
> 
> Right. I was hoping to keep this as a separate discussion since I
> think it's largely unrelated to the probe ordering issue, but we can
> talk about it here if you want.

As for me they are pretty much tired one to another. As reading EDID 
(even if it is just to read the panel ID) is one of the main issue with 
panel probe path. I just don't want to end up in a situation when we 
refactor aux_bus probe to fix the ordering/race issue and then we have 
to refactor it again for reading EDID.

> 
> There are a lot of open questions here and it's definitely hard to
> wrap your head around all of it. Maybe I'll just spam some thoughts
> and see if they all make sense together...

Thank you for the lengthy explanation. And I should be your pardon for 
partially ignoring DP/ dp bridges patches earlier.

> 
> 1. At the moment, there's no guarantee that a DP AUX Endpoint (AKA
> panel) will use pm_runtime() to power itself up enough to do an AUX
> transfer. At the moment the two eDP panels drivers I'm aware of use
> pm_runtime, but that's actually a fairly new behavior. I guess we'd
> have to codify it as "required" if we were going to rely on it.

* document it as a "required"

> 
> 2. In general, panels have powered themselves enough to read the EDID
> in their prepare() stage, which is equivalent to the bridge's
> pre_enable(). During some of my early patches to try to support EDID
> reading in ti-sn65dsi86 I actually relied upon it. It was like that in
> v3 [1]. Personally I see this as the "official" interface to power on
> the panel from the DP controller. As such I'm not sure we need to add
> pm_runtime() as an equivalent option.
> 
> 3. In the cover letter of v4 of my ti-sn65dsi86 EDID patch series I
> talked about why I switched to having EDID reading driven by the panel
> instead of powering on the panel (via pre_enable) and reading the EDID
> in the controller. One reason talked about there is that the "generic"
> eDP panel driver actually needs the EDID, or at least enough of it to
> get the panel ID, so that it can adjust its power sequence timings. If
> the EDID reading is completely handled by the DP driver and the panel
> can't do it then we'd need to figure out how to communicate it back.

I think with the current drm_bridge_connector-based code this should be 
handled properly. Anyway, it should be the panel, who reads the EDID, 
not the DP core. Actually just a random idea that just came to my mind. 
Maybe (!) we should break ties between msm dp core and the whole 
EDID/HPD/dp_panel story. In other words, split the whole DP EDID reading 
to the separate drm_bridge. Maybe I'm overengineering it here.

> 
> 4. In general, panels can be pretty persnickety about their power
> sequencing. As far as I've been able to tell, the official spec
> provides two things you can do:
> 
> 4a) You can power the panel up enough to do AUX transfers and then
> power it back off.
> 
> 4b) You can power the panel up enough to do AUX transfers, then finish
> powering it all the way up (turn on screen, backlight, etc). When you
> turn the screen off, if you follow the spec strictly, you're also
> _required_ to fully power the panel off. In other words, remove _all_
> power from the display including any power that would be needed to do
> AUX transfers.

Ugh. It's a pity that we can not leave AUX enabled forever, while doing 
all kinds of turning the screen off  and on again.

> 
> Now the generic eDP panel code doesn't currently follow the
> "strict"ness of the spec and I'm not actually sure if that's how the
> spec is intended to be interpreted anyway. There are two timing
> diagrams, though. One for "aux transfer only" and the other for
> "normal system operation". In the "normal system operation" the
> diagram doesn't allow for the backlight to ever go off and on again.
> 
> 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. I've also been
> trying to keep my eyes out to see if we need to do the same thing in
> generic eDP panel code, either for everyone or via some type of
> per-panel quirk. There's definitely a good reason to avoid the extra
> cycling if possible since powering panels off and on again often
> requires hundreds of milliseconds of delay in order to meet timing
> diagrams. ...and we do this if we ever change panel "modes".

Point noted.

> 
> ...OK, so why does this all matter? I guess my point here is I worry a
> little bit about saying that the DP controller code can willy nilly
> request the panel to be powered whenever it wants. If the DP
> controller was trying to hold the panel powered and then we _needed_
> to power the panel off then that would be bad. It doesn't mean we
> can't be careful about it, of course...
> 
> Said another way, in my mental model these three sequences are allowed:
> 
> s1) prepare, unprepare
> s2) prepare, enable, disable, unprepare
> s3) prepare, enable, disable, unprepare, prepare, enable, disable, unprepare
> 
> ...and this sequence is _not_ allowed:
> 
> s4) prepare, enable, disable, enable, disable, unprepare

A strange random question (for which there is probably an existing 
obvious answer somwewhere, 4 a.m. here).

Is there any reason why can't we drop prepare/unprepare for the eDP 
panels and use the following sequence;

- get_modes() = perform AUX-only transfer the first time we hit the 
function to read the EDID. return cached copy afterwards.

- a sequence of enable()/disable() calls doing a full powerup/powerdown?


> 
> ...and, in my mind, it's up to the panel driver to know whether in
> sequence s3) it has to _force_ power off between the unprepare and a
> prepare.
> 
> If pm_runtime() officially replaces prepare/unprepare then it's less
> obvious (in my mind) that we have to coordinate with enable().

I see

> 
> 5. In general I've been asserting that it should be up to the panel to
> power things on and drive all AUX transactions. ...but clearly my
> model isn't reality. We certainly do AUX transactions from the DP
> driver because the DP driver needs to know things about the connected
> device, like the number of lanes it has, the version of eDP it
> supports, and the available bit rates to name a few. Those things all
> work today by relying on the fact that pre-enable powers the panel on.
> It's pretty easy to say that reading the EDID (and I guess AUX
> backlight) is the odd one out. So right now I guess my model is:
> 
> 5a) If the panel code wants to access the AUX bus it can do so by
> powering itself on and then just doing an AUX transaction and assuming
> that the provider of the AUX bus can power itself on as needed.
> 
> 5b) If the DP code wants to access the AUX bus it should make sure
> that the next bridge's pre_enable() has been called. It can then
> assume that the device is powered on until the next bridge's
> post_disable() has been called.
> 
> So I guess tl;dr: I'm not really a huge fan of the DP driver powering
> the panel on by doing a pm_runtime_get() on it. I'd prefer to keep
> with the interface that we have to pre_enable() the panel to turn it
> on.

Again, thank for the explanation. Your concerns make more sense now.
As much as I hate writing docs, maybe we should put at least basic notes 
(regarding panel requirements) somewhere to the DP/DP AUX documentation?

> 
> 
> [1] https://lore.kernel.org/r/20210402152701.v3.8.Ied721dc895156046ac523baa55a71da241cd09c7@changeid/
> [2] https://lore.kernel.org/r/20210416223950.3586967-1-dianders@chromium.org/
> 
> 
>> BTW: as I'm slowly diving into DP vs eDP differences. Do we need to
>> write the EDID checksum like we do for DP?
> 
> Write the EDID checksum? I don't know what that means. You mean
> dp_panel_get_edid_checksum()? I'm not 100% sure, a quick glance seems
> to make me feel it has to do with DP compliance testing? I can dig
> more if need be. The generic EDID reading code already calculates the
> checksum, so unless you're doing some funny business you shouldn't
> need to check it again...

I was thinking about  dp_link_send_edid_checksum() / 
drm_dp_send_real_edid_checksum().

> 
> 
>> Do you have any good summary for eDP vs DP differences?
> 
> I don't. :( Mostly stuff here is me trying to grok bits out of what
> existing drivers were doing and trying to cross reference it with the
> eDP spec that I have (which I don't believe I can share,
> unfortunately).

I'll check if I can get DP and eDP specs on my own.

-- 
With best wishes
Dmitry