Date: Thu, 20 Jun 2024 18:07:20 -0700
From: Saravana Kannan <saravanak@...gle.com>
To: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Cc: Ulf Hansson <ulf.hansson@...aro.org>, Geert Uytterhoeven <geert+renesas@...der.be>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jiri Slaby <jirislaby@...nel.org>, 
	"Rafael J . Wysocki" <rafael@...nel.org>, Rob Herring <robh@...nel.org>, 
	Claudiu Beznea <claudiu.beznea.uj@...renesas.com>, Peng Fan <peng.fan@....com>, 
	linux-pm@...r.kernel.org, linux-serial@...r.kernel.org, 
	linux-renesas-soc@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Devarsh Thakkar <devarsht@...com>, 
	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH/RFC 0/3] pmdomain: renesas: rmobile-sysc: Remove serial
 console handling

On Wed, Jun 5, 2024 at 4:16 AM Tomi Valkeinen
<tomi.valkeinen@...asonboard.com> wrote:
>
> On 05/06/2024 13:53, Ulf Hansson wrote:
> > On Wed, 5 Jun 2024 at 12:41, Tomi Valkeinen
> > <tomi.valkeinen@...asonboard.com> wrote:
> >>
> >> Hi Ulf,
> >>
> >> On 05/06/2024 12:34, Ulf Hansson wrote:
> >>> + Tomi
> >>>
> >>> On Mon, 27 May 2024 at 14:41, Geert Uytterhoeven
> >>> <geert+renesas@...der.be> wrote:
> >>>>
> >>>>           Hi all,
> >>>>
> >>>> Since commit a47cf07f60dcb02d ("serial: core: Call
> >>>> device_set_awake_path() for console port"), the serial driver properly
> >>>> handles the case where the serial console is part of the awake path, and
> >>>> it looked like we could start removing special serial console handling
> >>>> from PM Domain drivers like the R-Mobile SYSC PM Domain driver.
> >>>> Unfortunately the devil is in the details, as usual...
> >>>>
> >>>> Earlycon relies on the serial port to be initialized by the firmware
> >>>> and/or bootloader.  Linux is not aware of any hardware dependencies that
> >>>> must be met to keep the port working, and thus cannot guarantee they
> >>>> stay met, until the full serial driver takes over.
> >>>>
> >>>> E.g. all unused clocks and unused PM Domains are disabled in a late
> >>>> initcall.  As this happens after the full serial driver has taken over,
> >>>> the serial port's clock and/or PM Domain are no longer deemed unused,
> >>>> and this is typically not a problem.
> >>>>
> >>>> However, if the serial port's clock or PM Domain is shared with another
> >>>> device, and that other device is runtime-suspended before the full
> >>>> serial driver has probed, the serial port's clock and/or PM Domain will
> >>>> be disabled inadvertently.  Any subsequent serial console output will
> >>>> cause a crash or system lock-up.  E.g. on R/SH-Mobile SoCs, the serial
> >>>> ports share their PM Domain with several other I/O devices.  After the
> >>>> use of pwm (Armadillo-800-EVA) or i2c (KZM-A9-GT) during early boot,
> >>>> before the full serial driver takes over, the PM Domain containing the
> >>>> early serial port is powered down, causing a lock-up when booted with
> >>>> "earlycon".
> >>>
> >>> Hi Geert,
> >>>
> >>> Thanks for the detailed description of the problem! As pointed out in
> >>> regards to another similar recent patch [1], this is indeed a generic
> >>> problem, not limited to the serial console handling.
> >>>
> >>> At Linaro Connect a few weeks ago I followed up with Saravana from the
> >>> earlier discussions at LPC last fall. We now have a generic solution
> >>> for genpd drafted on plain paper, based on fw_devlink and the
> >>> ->sync_state() callback. I am currently working on the genpd series,
> >>> while Saravana will re-spin the series (can't find the link to the
> >>> last version) for the clock framework. Ideally, we want these things
> >>> to work in a very similar way.
> >>>
> >>> That said, allow me to post the series for genpd in a week or two to
> >>> see if it can solve your problem too, for the serial console.
> >>
> >> Both the genpd and the clock solutions will make suppliers depend on all
> >> their consumers to be probed, right?
> >>
> >> I think it is a solution, and should be worked on, but it has the
> >> drawback that suppliers that have consumers that will possibly never be
> >> probed, will also never be able to turn off unused resources.
> >>
> >> This was specifically the case with the TI ti-sci pmdomain case I was
> >> looking at: the genpd driver (ti_sci_pm_domains.c) provides a lot of
> >> genpds for totally unrelated devices, and so if, e.g., you don't have or
> >> don't want to load a driver for the GPU, all PDs are affected.
> >>
> >> Even here the solutions you mention will help: instead of things getting
> >> broken because genpds get turned off while they are actually in use, the
> >> genpds will be kept enabled, thus fixing the breakage. Unfortunately,
> >> they'll be kept enabled forever.
> >>
> >> I've been ill for quite a while so I haven't had the chance to look at
> >> this more, but before that I was hacking around a bit with something I
> >> named .partial_sync_state(). .sync_state() gets called when all the
> >> consumers have probed, but .partial_sync_state() gets called when _a_
> >> consumer has been probed.
> >>
> >> For the .sync_state() things are easy for the driver, as it knows
> >> everything related has been probed, but for .partial_sync_state() the
> >> driver needs to track resources internally. .partial_sync_state() will
> >> tell the driver that a consumer device has probed, the driver can then
> >> find out which specific resources (genpds in my case) that consumer
> >> refers to, and then... Well, that's how far I got with my hacks =).
> >>
> >> So, I don't know if this .partial_sync_state() can even work, but I
> >> think we do need something more on top of the .sync_state().
> >
> > Thanks for the update!
> >
> > You certainly have a point, but rather than implementing some platform
> > specific method, I think we should be able enforce the call to
> > ->sync_state(), based upon some condition/timeout - and even if all
> > consumers haven't been probed.
>
> Hmm, I think that was already implemented in some of the serieses out
> there (or even in mainline already?), as I remember doing some
> experiments with it. I don't like it much, though.
>
> With a simple timeout, it'll always be just a bit too early for some
> user (nfs mount took a bit more time than expected -> board frozen).
>
> The only condition I can see that would somewhat work is a manual
> trigger from the userspace. The boot scripts could then signal the
> kernel when all the modules have been loaded and probably a suitable,
> platform/use case specific amount of time has passed to allow the
> drivers to probe.

This is also already supported in mainline.

Devices with sync_state() implementations (once Ulf adds it) will have
a state_synced file in sysfs. It shows where it has been called yet or
not. But you can also echo 1 into it to force the sync_state()
callback (only if it hasn't been called already). So, yeah, all
methods of handling this are available if you implement the
sync_state() callback.

By default it's all strict (wait till all consumers probe
successfully). But you can set it to timeout (fw_devlink.sync_state).
And you also have the option I mentioned above that you can use with
both cases.

-Saravana

>
> It just feels a bit too much of a "let's hope this work" approach.
>
> That said, the timeout/condition is probably acceptable for many cases,
> where turning off a resource forcefully will just result in, say, a
> temporarily blanked display, or something else that gets fixed if and
> when the proper driver is probed.
>
> Unfortunately, here with the case I have, the whole board gets halted if
> the display subsystem genpd is turned off and the display driver is
> loaded after that.
>
>   Tomi
>

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