timekeeping: Evaluate system_counterval_t.cs_id instead of .cs

Clocksource pointers can be problematic to obtain for drivers which are not
clocksource drivers themselves. In particular, the RFC virtio_rtc driver
[1] would require a new helper function to obtain a pointer to the ARM
Generic Timer clocksource. The ptp_kvm driver also required a similar
workaround.

Address this by evaluating the clocksource ID, rather than the clocksource
pointer, of struct system_counterval_t. By this, setting the clocksource
pointer becomes unneeded, and get_device_system_crosststamp() callers will
no longer need to supply clocksource pointers.

All relevant clocksource drivers provide the ID, so this change is not
changing the behaviour.

[1] https://lore.kernel.org/lkml/20231218073849.35294-1-peter.hilber@opensynergy.com/Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240201010453.2212371-7-peter.hilber@opensynergy.com

include/linux/timekeeping.h

@@ -268,13 +268,13 @@ struct system_device_crosststamp {
 };
 
 /**
- * struct system_counterval_t - system counter value with the pointer to the
+ * struct system_counterval_t - system counter value with the ID of the
  *				corresponding clocksource
  * @cycles:	System counter value
- * @cs:		Clocksource corresponding to system counter value. Used by
- *		timekeeping code to verify comparibility of two cycle values
- * @cs_id:	Clocksource ID corresponding to system counter value. To be
- *		used instead of cs in the future.
+ * @cs:		Clocksource corresponding to system counter value. Timekeeping
+ *		code now evaluates cs_id instead.
+ * @cs_id:	Clocksource ID corresponding to system counter value. Used by
+ *		timekeeping code to verify comparability of two cycle values.
  *		The default ID, CSID_GENERIC, does not identify a specific
  *		clocksource.
  */

kernel/time/timekeeping.c

@@ -1232,11 +1232,12 @@ int get_device_system_crosststamp(int (*get_time_fn)
 			return ret;
 
 		/*
-		 * Verify that the clocksource associated with the captured
-		 * system counter value is the same as the currently installed
-		 * timekeeper clocksource
+		 * Verify that the clocksource ID associated with the captured
+		 * system counter value is the same as for the currently
+		 * installed timekeeper clocksource
 		 */
-		if (tk->tkr_mono.clock != system_counterval.cs)
+		if (system_counterval.cs_id == CSID_GENERIC ||
+		    tk->tkr_mono.clock->id != system_counterval.cs_id)
 			return -ENODEV;
 		cycles = system_counterval.cycles;