Message-ID: <b9c97dc4-b5d6-4886-967c-6b7d331d27a7@tuxon.dev>
Date: Tue, 11 Jun 2024 07:52:03 +0300
From: claudiu beznea <claudiu.beznea@...on.dev>
To: Geert Uytterhoeven <geert+renesas@...der.be>,
 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>, Saravana Kannan <saravanak@...gle.com>
Cc: 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
Subject: Re: [PATCH/RFC 0/3] pmdomain: renesas: rmobile-sysc: Remove serial
 console handling

Hi, Geert,

On 27.05.2024 15:41, Geert Uytterhoeven 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".
> 
> 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.
> 
> 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,

For the record, I've tested this series on RZ/G3S. All good with it.
If any, you can add my:

Tested-by: Claudiu Beznea <claudiu.beznea.uj@...renesas.com>

Thank you,
Claudiu Beznea

>   - 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(-)
> 

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