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Date: Tue, 30 Apr 2024 16:28:11 +0200
From: Peter Hilber <peter.hilber@opensynergy.com>
Subject: Re: [PATCH v7 01/12] timekeeping: Add base clock properties in
 clocksource structure
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On 30.04.24 10:52, lakshmi.sowjanya.d@intel.com wrote:
> From: Lakshmi Sowjanya D <lakshmi.sowjanya.d@intel.com>
> 
> Add base clock hardware abstraction in clocksource structure.
> 
> Provide generic functionality in convert_base_to_cs() to convert base
> clock timestamps to system clocksource without requiring architecture
> specific parameters.
> 
> This is intended to replace convert_art_to_tsc() and
> convert_art_ns_to_tsc() functions which are specific to convert ART
> (Always Running Timer) time to the corresponding TSC value.
> 
> Add the infrastructure in get_device_system_crosststamp().
> 
> Co-developed-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Co-developed-by: Christopher S. Hall <christopher.s.hall@intel.com>
> Signed-off-by: Christopher S. Hall <christopher.s.hall@intel.com>
> Signed-off-by: Lakshmi Sowjanya D <lakshmi.sowjanya.d@intel.com>
> ---
>  include/linux/clocksource.h | 27 +++++++++++++++++++++++++
>  include/linux/timekeeping.h |  2 ++
>  kernel/time/timekeeping.c   | 39 +++++++++++++++++++++++++++++++++++--
>  3 files changed, 66 insertions(+), 2 deletions(-)
> 
[...]
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index b58dffc58a8f..4e5e931e766a 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -1193,6 +1193,42 @@ static bool timestamp_in_interval(u64 start, u64 end, u64 ts)
>  	return false;
>  }
>  
> +static bool convert_clock(u64 *val, u32 numerator, u32 denominator)
> +{
> +	u64 rem, res;
> +
> +	if (!numerator || !denominator)
> +		return false;
> +
> +	res = div64_u64_rem(*val, denominator, &rem) * numerator;
> +	*val = res + div_u64(rem * numerator, denominator);
> +	return true;
> +}
> +
> +static bool convert_base_to_cs(struct system_counterval_t *scv)
> +{
> +	struct clocksource *cs = tk_core.timekeeper.tkr_mono.clock;
> +	struct clocksource_base *base = cs->base;
> +	u32 num, den;
> +
> +	/* The timestamp was taken from the time keeper clock source */
> +	if (cs->id == scv->cs_id)
> +		return true;
> +
> +	/* Check whether cs_id matches the base clock */
> +	if (!base || base->id != scv->cs_id)
> +		return false;
> +
> +	num = scv->use_nsecs ? cs->freq_khz : base->numerator;
> +	den = scv->use_nsecs ? USEC_PER_SEC : base->denominator;
> +
> +	if (!convert_clock(&scv->cycles, num, den))
> +		return false;
> +
> +	scv->cycles += base->offset;
> +	return true;
> +}
> +
>  /**
>   * get_device_system_crosststamp - Synchronously capture system/device timestamp
>   * @get_time_fn:	Callback to get simultaneous device time and
> @@ -1238,8 +1274,7 @@ int get_device_system_crosststamp(int (*get_time_fn)
>  		 * system counter value is the same as for the currently
>  		 * installed timekeeper clocksource
>  		 */
> -		if (system_counterval.cs_id == CSID_GENERIC ||
> -		    tk->tkr_mono.clock->id != system_counterval.cs_id)
> +		if (!convert_base_to_cs(&system_counterval))
>  			return -ENODEV;

AFAIU the error condition

	system_counterval.cs_id == CSID_GENERIC

is silently dropped by this patch, but shouldn't be. 

get_device_system_crosststamp() can only check the identity of a
clocksource (base) for non-generic ids.

Regards,

Peter