Date: Fri, 21 Jun 2024 11:49:10 +0300
From: Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>,
 Saravana Kannan <saravanak@...gle.com>
Cc: 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

On 21/06/2024 10:07, Geert Uytterhoeven wrote:
> 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?

I don't know the exact HW level details. It may be a HW bug or possibly 
a firmware issue. But what I see is simple:

If the display subsystem is powered and a video output is enabled, 
turning off the PD causes the display subsystem to go to a bad state, 
after which the next register access will halt the cpu.

This happens quite easily if the bootloader has enabled a display: when 
the kernel's tidss driver probes, the device framework will make sure 
the PD is enabled (which is fine, it's basically a no-op). But if the 
tidss returns an error, like EPROBE_DEFER, the device framework will 
disable the PD. When the tidss driver probes again later, it will cause 
a halt at a register access.

  Tomi

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