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

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index ca234fa4cc046341f6bf0837483e0f6bed8312c5..3538c5bdf9ee3263dd16adca8fe43142b468a870 100644 (file)
--- 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 266d02809dbb1dceabfb2a05276385c6a3be9b0c..0ff065c5d25b3ac1e72a45b24f56221d9eec03cc 100644 (file)
--- 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;