[RFC PATCH v2 5/7] timekeeping: Evaluate system_counterval_t.cs_id instead of .cs

[Peter Hilber]

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 it will be dropped from struct
system_counterval_t in the future. By this, get_device_system_crosststamp()
callers (such as virtio_rtc and ptp_kvm) will no longer need to supply
clocksource pointers.

This change should not alter any behavior, as the struct
system_counterval_t clocksource ID is already being set wherever the
clocksource pointer is set. get_device_system_crosststamp() will now fail
if the clocksource has id CSID_GENERIC, but all currently relevant
clocksources have a custom clocksource id.

[1] https://lore.kernel.org/lkml/20230818012014.212155-1-peter.hilber@opensynergy.com/

Signed-off-by: Peter Hilber <peter.hilber at opensynergy.com>
---

Notes:
    v2:
    
    - Refer to clocksource IDs as such in comments (Thomas Gleixner).
    
    - Update comments which were still referring to clocksource pointers.

 include/linux/timekeeping.h | 10 +++++-----
 kernel/time/timekeeping.c   |  9 +++++----
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index 74dc7c8b036f..75e957171bd5 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -267,13 +267,13 @@ 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.
  */
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 266d02809dbb..0ff065c5d25b 100644
--- a/kernel/time/timekeeping.c
+++ b/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;
 
-- 
2.40.1