Merge branch 'ptp_vclock-fixes'

Vladimir Oltean says:

====================
ptp_vclock fixes

While I was intending to test something else related to PTP in net-next
I noticed any time I would run ptp4l on an interface, the kernel would
print "ptp: physical clock is free running" and ptp4l would exit with an
error code.

I then found Jeongjun Park's patch and subsequent explanation provided
to Jakub's question, specifically related to the code which introduced
the breakage I am seeing.
https://lore.kernel.org/netdev/CAO9qdTEjQ5414un7Yw604paECF=6etVKSDSnYmZzZ6Pg3LurXw@mail.gmail.com/

I had to look at the original issue that prompted Jeongjun Park's patch,
and provide an alternative fix for it. Patch 1/2 in this set contains a
logical revert plus the alternative fix, squashed into one.

Patch 2/2 fixes another issue which was confusing during debugging/
characterization, namely: "ok, the kernel clearly thinks that any
physical clock is free-running after this change (despite there being no
vclocks), but why would ptp4l fail to create the clock altogether? Why
not just fail to adjust it?"

By reverting (locally) Jeongjun Park's commit, I could reproduce
the reported lockdep splat using the commands from patch 1/2's commit
message, and this goes away with the reworked implementation.
====================

Link: https://patch.msgid.link/20250613174749.406826-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

drivers/ptp/ptp_clock.c

@@ -121,7 +121,8 @@ static int ptp_clock_adjtime(struct posix_clock *pc, struct __kernel_timex *tx)
 	struct ptp_clock_info *ops;
 	int err = -EOPNOTSUPP;
 
-	if (ptp_clock_freerun(ptp)) {
+	if (tx->modes & (ADJ_SETOFFSET | ADJ_FREQUENCY | ADJ_OFFSET) &&
+	    ptp_clock_freerun(ptp)) {
 		pr_err("ptp: physical clock is free running\n");
 		return -EBUSY;
 	}

drivers/ptp/ptp_private.h

@@ -98,7 +98,27 @@ static inline int queue_cnt(const struct timestamp_event_queue *q)
 /* Check if ptp virtual clock is in use */
 static inline bool ptp_vclock_in_use(struct ptp_clock *ptp)
 {
-	return !ptp->is_virtual_clock;
+	bool in_use = false;
+
+	/* Virtual clocks can't be stacked on top of virtual clocks.
+	 * Avoid acquiring the n_vclocks_mux on virtual clocks, to allow this
+	 * function to be called from code paths where the n_vclocks_mux of the
+	 * parent physical clock is already held. Functionally that's not an
+	 * issue, but lockdep would complain, because they have the same lock
+	 * class.
+	 */
+	if (ptp->is_virtual_clock)
+		return false;
+
+	if (mutex_lock_interruptible(&ptp->n_vclocks_mux))
+		return true;
+
+	if (ptp->n_vclocks)
+		in_use = true;
+
+	mutex_unlock(&ptp->n_vclocks_mux);
+
+	return in_use;
 }
 
 /* Check if ptp clock shall be free running */