[RFC PATCH v2 1/7] timekeeping: Add clocksource ID to struct system_counterval_t

[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.

Add a clocksource ID member to struct system_counterval_t, which in the
future shall identify the clocksource, and shall replace the struct
clocksource * member. By this, get_device_system_crosststamp() callers
(such as virtio_rtc and ptp_kvm) will be able to supply easily accessible
clocksource ids instead of clocksource pointers.

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

Signed-off-by: Peter Hilber <peter.hilber@xxxxxxxxxxxxxxx>
---

Notes:
    v2:
    
    - Refer to clocksource IDs as such in comments (Thomas Gleixner).

 include/linux/timekeeping.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index fe1e467ba046..74dc7c8b036f 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -272,10 +272,15 @@ struct system_device_crosststamp {
  * @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.
+ *		The default ID, CSID_GENERIC, does not identify a specific
+ *		clocksource.
  */
 struct system_counterval_t {
 	u64			cycles;
 	struct clocksource	*cs;
+	enum clocksource_ids	cs_id;
 };
 
 /*
-- 
2.40.1