linux-kernel - Re: [PATCH/RFC 1/3] earlycon: Export clock and PM Domain info from FDT

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Date: Wed, 29 May 2024 11:01:25 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Geert Uytterhoeven <geert+renesas@...der.be>
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>, 
	Saravana Kannan <saravanak@...gle.com>, 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 1/3] earlycon: Export clock and PM Domain info from FDT

On Mon, May 27, 2024 at 2:41 PM Geert Uytterhoeven
<geert+renesas@...der.be> wrote:
> 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.
>
> Provide 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.
> Note that as this is done during early boot-up, the device_node
> structure pointing to the earlycon console is not yet created, so this
> has to resort to raw property data.
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>

> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -954,6 +954,16 @@ static const bool earlycon_acpi_spcr_enable EARLYCON_USED_OR_UNUSED;
>  static inline int setup_earlycon(char *buf) { return 0; }
>  #endif
>
> +#ifdef CONFIG_OF_EARLY_FLATTREE

This should include a check  for CONFIG_SERIAL_EARLYCON.

> +extern const __be32 *earlycon_clocks, *earlycon_power_domains;
> +extern int earlycon_clocks_ncells, earlycon_power_domains_ncells;
> +#else
> +#define earlycon_clocks                        NULL
> +#define earlycon_clocks_ncells         0
> +#define earlycon_power_domains         NULL
> +#define earlycon_power_domains_ncells  0
> +#endif
> +
>  /* Variant of uart_console_registered() when the console_list_lock is held. */
>  static inline bool uart_console_registered_locked(struct uart_port *port)
>  {

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68korg

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds