Date: Fri, 21 Jun 2024 09:07:19 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Saravana Kannan <saravanak@...gle.com>
Cc: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>, Ulf Hansson <ulf.hansson@...aro.org>, 
	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

Hi Saravana,

On Fri, Jun 21, 2024 at 3:08 AM Saravana Kannan <saravanak@...gle.com> wrote:
> 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:
> > >> On 05/06/2024 12:34, Ulf Hansson wrote:
> > >>> On Mon, 27 May 2024 at 14:41, Geert Uytterhoeven
> > >>> <geert+renesas@...der.be> wrote:
> > >>>> 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.

Let's call this "Case A".

> > >>>>
> > >>>> 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".

Let's call this "Case B".

> > >>>
> > >>> 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.

So the idea is to disable unused genpds and clocks from the genpd
resp. clock's driver .sync_state() callback, instead of from a late
initcall?  That would indeed solve issues related to "Case A".

However, how to solve "Case B"? Ignore disabling genpds or clocks
before .sync_state() callback() has been called?
That would cause issues for cases where the clock must be disabled,
cfr.
    "[PATCH RFC 0/3] Add clk_disable_unprepare_sync()"
    https://lore.kernel.org/all/20240131160947.96171-1-biju.das.jz@bp.renesas.com/
    "[PATCH v3 0/3] Add clk_poll_disable_unprepare()"
    https://lore.kernel.org/linux-renesas-soc/20240318110842.41956-1-biju.das.jz@bp.renesas.com/

> > 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: Do you have more details? The genpd must be controlling something
critical that must never be turned off, or perhaps the display driver
lacks some initialization?

Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Powered by blists - more mailing lists