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@xxxxxxxxxxxxxx>
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(-)
>
