Re: [PATCH/RFC 0/3] pmdomain: renesas: rmobile-sysc: Remove serial console handling

[Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>]

Hi Ulf,

On 05/06/2024 12:34, Ulf Hansson wrote:
> + Tomi
> 
> On Mon, 27 May 2024 at 14:41, Geert Uytterhoeven
> <geert+renesas@glider.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().

  Tomi

> 
> Kind regards
> Uffe
> 
> [1]
> https://lore.kernel.org/linux-arm-kernel/CAPDyKFqShuq98qV5nSPzSqwLLUZ7LxLvp1eihGRBkU4qUKdWwQ@mail.gmail.com/
> 
>>
>> This RFC patch series aims to provide a mechanism for handling this, and
>> to fix it for the PM Domain case:
>>    1. The first patch provides a mechanism to let the clock and/or PM
>>       Domain subsystem or drivers handle this, by exporting the clock and
>>       PM Domain dependencies for the serial port, as available in the
>>       system's device tree,
>>    2. The second patch introduces a new flag to handle a PM domain that
>>       must be kept powered-on during early boot, and by setting this flag
>>       if the PM Domain contains the serial console (originally I handled
>>       this inside rmobile-sysc, but it turned out to be easy to
>>       generalize this to other platforms in the core PM Domain code).
>>    3. The third patch removes the no longer needed special console
>>       handling from the R-Mobile SYSC PM Domain driver.
>>
>> I did not fix the similar clock issue, as it is more complex (there can
>> be multiple clocks, and each clock provider can have its own value of
>> #clock-cells), and I do not need it for Renesas ARM platforms.
> 
> I will defer to Sarvana here, but ideally his series for the clock
> framework should solve this case too.
> 
>>
>> This has been tested on the APE6-EVM, Armadillo-800-EVA, and KZM-A9-GT
>> development boards, with and without earlycon, including s2ram with and
>> without no_console_suspend.
>>
>> Notes:
>>    - This should not be needed on RZ/G3S, where each serial port device
>>      has its own PM Domain,
>>    - drivers/clk/imx/clk.c and drivers/pmdomain/imx/scu-pd.c have special
>>      handling for the of_stdout device, but is probably not affected, as
>>      each serial port seems to share its PM Domain only with the serial
>>      port's clock controller.
>>
>> Thanks for your comments!
>>
>> Geert Uytterhoeven (3):
>>    earlycon: Export clock and PM Domain info from FDT
>>    pmdomain: core: Avoid earlycon power-down
>>    pmdomain: renesas: rmobile-sysc: Remove serial console handling
>>
>>   drivers/pmdomain/core.c                 | 24 ++++++++++++++++--
>>   drivers/pmdomain/renesas/rmobile-sysc.c | 33 +------------------------
>>   drivers/tty/serial/earlycon.c           | 14 ++++++++++-
>>   include/linux/pm_domain.h               |  4 +++
>>   include/linux/serial_core.h             | 10 ++++++++
>>   5 files changed, 50 insertions(+), 35 deletions(-)
>>
>> --
> 
> Kind regards
> Uffe