Date: Tue, 21 May 2024 12:29:07 +0200
From: Ulf Hansson <ulf.hansson@...aro.org>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Abel Vesa <abel.vesa@...aro.org>, Saravana Kannan <saravanak@...gle.com>, 
	Stephen Boyd <sboyd@...nel.org>, Nishanth Menon <nm@...com>, Tero Kristo <kristo@...nel.org>, 
	Santosh Shilimkar <ssantosh@...nel.org>, Dave Gerlach <d-gerlach@...com>, J Keerthy <j-keerthy@...com>, 
	Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Santosh Shilimkar <santosh.shilimkar@...cle.com>, linux-arm-kernel@...ts.infradead.org, 
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	devicetree@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	Devarsh Thakkar <devarsht@...com>
Subject: Re: [PATCH RFC 2/2] pmdomain: ti-sci: Support retaining PD boot time state

On Fri, 10 May 2024 at 12:19, Tomi Valkeinen
<tomi.valkeinen@...asonboard.com> wrote:
>
> Hi,
>
> On 03/05/2024 16:45, Ulf Hansson wrote:
> > + Abel, Saravanna, Stephen
> >
> > On Mon, 15 Apr 2024 at 19:17, Tomi Valkeinen
> > <tomi.valkeinen@...asonboard.com> wrote:
> >>
> >> On 15/04/2024 19:00, Tomi Valkeinen wrote:
> >>> Add a new flag, TI_SCI_PD_KEEP_BOOT_STATE, which can be set in the dts
> >>> when referring to power domains. When this flag is set, the ti-sci
> >>> driver will check if the PD is currently enabled in the HW, and if so,
> >>> set the GENPD_FLAG_ALWAYS_ON flag so that the PD will stay enabled.
> >>>
> >>> The main issue I'm trying to solve here is this:
> >>>
> >>> If the Display Subsystem (DSS) has been enabled by the bootloader, the
> >>> related PD has also been enabled in the HW. When the tidss driver
> >>> probes, the driver framework will automatically enable the PD. While
> >>> executing the probe function it is very common for the probe to return
> >>> EPROBE_DEFER, and, in rarer cases, an actual error. When this happens
> >>> (probe() returns an error), the driver framework will automatically
> >>> disable the related PD.
> >>>
> >>> Powering off the PD while the DSS is enabled and displaying a picture
> >>> will cause the DSS HW to enter a bad state, from which (afaik) it can't
> >>> be woken up except with full power-cycle. Trying to access the DSS in
> >>> this state (e.g. when retrying the probe) will usually cause the board
> >>> to hang sooner or later.
> >>>
> >>> Even if we wouldn't have this board-hangs issue, it's nice to be able to
> >>> keep the DSS PD enabled: we want to keep the DSS enabled when the
> >>> bootloader has enabled the screen. If, instead, we disable the PD at the
> >>> first EPROBE_DEFER, the screen will (probably) go black.
> >>
> >> A few things occurred to me. The driver is supposed to clear the
> >> GENPD_FLAG_ALWAYS_ON when the driver has probed successfully. There are
> >> two possible issues with that:
> >>
> >> - Afaics, there's no API to do that, and currently I just clear the bit
> >> in genpd->flags. There's a clear race there, so some locking would be
> >> required.
> >>
> >> - This uses the GENPD_FLAG_ALWAYS_ON flag to say "PD is always on, until
> >> the driver has started". If the PD would have GENPD_FLAG_ALWAYS_ON set
> >> for other reasons, the driver would still go and clear the flag, which
> >> might break things.
> >>
> >> Also, unrelated to the above and not a problem in practice at the very
> >> moment, but I think clocks should also be dealt with somehow. Something,
> >> at early-ish boot stage, should mark the relevant clocks as in use, so
> >> that there's no chance they would be turned off when the main kernel has
> >> started (the main display driver is often a module).
> >>
> >> It would be nice to deal with all the above in a single place. I wonder
> >> if the tidss driver itself could somehow be split into two parts, an
> >> early part that would probe with minimal dependencies, mainly to reserve
> >> the core resources without doing any kind of DRM init. And a main part
> >> which would (somehow) finish the initialization at a later point, when
> >> we have the filesystem (for firmware) and the other bridge/panel drivers
> >> have probed.
> >>
> >> That can be somewhat achieved with simplefb or simpledrm, though, but we
> >> can't do any TI DSS specific things there, and it also creates a
> >> requirement to have either of those drivers built-in, and the related DT
> >> nodes to be added.
> >
> > Without going into too much detail, this and similar problems have
> > been discussed in the past. With the fw_devlink and the ->sync_state()
> > callback we are getting closer to a solution, but for genpd a solution
> > is still pending.
> >
> > If you want to read up on earlier discussions and join us moving
> > forward, that would be great. The last attempt for genpd to move this
> > forward was posted by Abel Vesa:
> > https://lore.kernel.org/linux-pm/20230621144019.3219858-1-abel.vesa@linaro.org/
> >
> > Beyond that, we have also discussed various solutions at the last LPC
> > in Richmond. I think the consensus at that point was that Saravana
> > targeted to post something for clocks - and when that was done, we
> > should do the similar thing for genpd. Anyway, I have looped them into
> > this thread, so they can share any updates on their side of the
> > matter.
>
> If I understand the series correctly, it has an issue at least for this
> case/platform.
>
> The devlinks are between the consumer devices and the PD provider
> device. TI SCI PD provider has quite a lot of PDs, and all the consumers
> would have to be probed before any of the PDs could be disabled. So, to
> get the display PD disabled, I would have to load, e.g., the GPU driver
> (which I don't even have).
>
> I believe this is the case for the clocks also.
>
> Perhaps that can be considered a feature, but I fear that in practice it
> would mean that most of the time for most users all the boot-time
> enabled powerdomains would be always on.
>
> Nevertheless, I believe the series would fix the issue mentioned in this
> patch, so I'll see if I can get the series working on the TI platform to
> get a bit more experience on this whole issue.

[...]

Just to share a brief update on this matter. I have had an offlist
chat with Saravana and some Qcom guys about this topic. In particular
we looked closer at some implementation details.

Allow me a few weeks, then I will post a series for genpd to implement
the above. I will keep you posted and would appreciate reviews and
tests, of course.

Kind regards
Uffe

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