[PATCH v3 6/8] timekeeping: Evaluate system_counterval_t.cs_id instead of .cs

From: Peter Hilber
Date: Wed Jan 31 2024 - 20:13:42 EST


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/20231218073849.35294-1-peter.hilber@xxxxxxxxxxxxxxx/

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

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 ca234fa4cc04..3538c5bdf9ee 100644
--- a/include/linux/timekeeping.h
+++ b/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.
*/
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