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

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

On 13.11.23 14:03, Sascha Hauer wrote:
> 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.

Those SoCs can still implement CONFIG_DEBUG_LL and just skip the
initialization step.

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

If clk turns out to be not enabled, system would hang on e.g. i.MX.

This is a mere debugging measure for SoCs with complex clock controllers
backed by firmware, so I think having to enable CONFIG_DEBUG_LL to use
is acceptable.

Cheers,
Ahmad


> 
> 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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