Re: [PATCH 1/2] console: provide best-effort clk_get_for_console helper

[Sascha Hauer <sha@pengutronix.de>]

On Thu, Nov 09, 2023 at 12:47:06PM +0100, Ahmad Fatoum wrote:
> From: Ahmad Fatoum <ahmad@a3f.at>
> 
> clk_get will return -EPROBE_DEFER if clock provider hasn't yet been
> probed. In a system with deep probe enabled, dependencies are probed
> on demand, so a -EPROBE_DEFER return is final and the console probe
> will never succeed.
> 
> CONFIG_DEBUG_LL is often used to debug this, but because the low-level
> console is not interactive, it's a bit cumbersome. Improve upon this a
> bit, by providing a clk_get_for_console helper that returns NULL if and
> only if we are sure that a clock provider will not be probed.
> 
> In that case, the driver can skip code paths initialization code and
> baud rate setting dependent on having access to the clock and still
> register a console that reuses what was set up by CONFIG_DEBUG_LL.
> 
> Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
> ---
>  include/console.h   | 26 ++++++++++++++++++++++++++
>  include/linux/clk.h | 21 +++++++++++++++++++++
>  2 files changed, 47 insertions(+)
> 
> diff --git a/include/console.h b/include/console.h
> index 586b68f73301..b8c901801e9f 100644
> --- a/include/console.h
> +++ b/include/console.h
> @@ -8,6 +8,7 @@
>  #define _CONSOLE_H_
>  
>  #include <param.h>
> +#include <linux/clk.h>
>  #include <linux/list.h>
>  #include <driver.h>
>  #include <serdev.h>
> @@ -208,4 +209,29 @@ static inline void console_ctrlc_allow(void) { }
>  static inline void console_ctrlc_forbid(void) { }
>  #endif
>  
> +/**
> + * clk_get_for_console - get clock, ignoring known unavailable clock controller
> + * @dev: device for clock "consumer"
> + * @id: clock consumer ID
> + *
> + * Return: a struct clk corresponding to the clock producer, a
> + * valid IS_ERR() condition containing errno or NULL if it could
> + * be determined that the clock producer will never be probed in
> + * absence of modules. The NULL return allows serial drivers to
> + * skip clock handling and rely on CONFIG_DEBUG_LL.
> + */
> +static inline struct clk *clk_get_for_console(struct device *dev, const char *id)
> +{
> +	struct clk *clk;
> +
> +	if (!IS_ENABLED(CONFIG_DEBUG_LL))
> +		return clk_get(dev, id);

There are several SoCs out there where the UART is enabled by the ROM
already, on these we don't need to have CONFIG_DEBUG_LL enabled for a
working UART.

Maybe testing for the UART enable bit in probe() would be a better
indication that the UART is already in a working state?

Sascha

> +
> +	clk = clk_get_if_available(dev, id);
> +	if (clk == NULL)
> +		dev_warn(dev, "couldn't get clock (ignoring)\n");
> +
> +	return clk;
> +}
> +
>  #endif
> diff --git a/include/linux/clk.h b/include/linux/clk.h
> index 61b4ff3127cf..f505f18a75c0 100644
> --- a/include/linux/clk.h
> +++ b/include/linux/clk.h
> @@ -14,6 +14,7 @@
>  #include <linux/spinlock.h>
>  #include <linux/stringify.h>
>  #include <linux/container_of.h>
> +#include <deep-probe.h>
>  #include <xfuncs.h>
>  
>  struct device;
> @@ -982,4 +983,24 @@ static inline struct clk *clk_get_enabled(struct device *dev, const char *id)
>  	return clk;
>  }
>  
> +/**
> + * clk_get_if_available - get clock, ignoring known unavailable clock controller
> + * @dev: device for clock "consumer"
> + * @id: clock consumer ID
> + *
> + * Return: a struct clk corresponding to the clock producer, a
> + * valid IS_ERR() condition containing errno or NULL if it could
> + * be determined that the clock producer will never be probed in
> + * absence of modules.
> + */
> +static inline struct clk *clk_get_if_available(struct device *dev, const char *id)
> +{
> +	struct clk *clk = clk_get(dev, id);
> +
> +	if (clk == ERR_PTR(-EPROBE_DEFER) && deep_probe_is_supported())
> +		return NULL;
> +
> +	return clk;
> +}
> +
>  #endif
> -- 
> 2.39.2
> 
> 
> 

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