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

[Ulf Hansson]

+ Tomi

On Mon, 27 May 2024 at 14:41, Geert Uytterhoeven
<geert+renesas@xxxxxxxxx> 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.

Kind regards
Uffe

[1]
https://lore.kernel.org/linux-arm-kernel/CAPDyKFqShuq98qV5nSPzSqwLLUZ7LxLvp1eihGRBkU4qUKdWwQ@xxxxxxxxxxxxxx/

>
> 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