alpha: run the remote RTC access in a worker, not an IPI callback

On Marvel the CMOS clock is only reachable from the boot cpu, so
remote_read_time() and remote_set_time() bounce the access there with
smp_call_function_single(), whose callback runs in hard interrupt
context.

alpha_rtc_read_time() calls mc146818_get_time() with a 10 ms timeout.
That waits out the RTC update cycle in mc146818_avoid_UIP(), which drops
rtc_lock and udelay()s 100 us at a time until the update completes or
the timeout expires:

	for (i = 0; UIP_RECHECK_LOOPS_MS(i) < timeout; i++) {
		spin_lock_irqsave(&rtc_lock, flags);
		...
		if (CMOS_READ(RTC_FREQ_SELECT) & RTC_UIP) {
			spin_unlock_irqrestore(&rtc_lock, flags);
			udelay(UIP_RECHECK_DELAY);
			continue;
		}

So a clock read from a non-boot cpu can spin for up to 10 ms in hard
interrupt context on the boot cpu, while the cpu that sent the request
spins in smp_call_function_single() waiting for it to finish.

mc146818_set_time() does not poll, but it takes rtc_lock too, and
rtc_lock is a spinlock_t.  Only raw spinlocks may be taken in hard
interrupt context, so lockdep reports the write path as soon as a
non-boot cpu sets the clock:

  [ BUG: Invalid wait context ]
  -----------------------------
  swapper/0/0 is trying to lock:
  fffffc0003690470 (rtc_lock){....}-{3:3}, at: mc146818_set_time+0x74/0x450
  other info that might help us debug this:
  context-{2:2}
  no locks held by swapper/0/0.
  stack backtrace:
  CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 7.2.0-rc1 #1 NONE
  Trace:
  [<fffffc000102ebb0>] dump_stack+0x28/0x44
  [<fffffc000110efcc>] __lock_acquire+0xb0c/0x1060
  [<fffffc000110f5f0>] lock_acquire.part.0+0xd0/0x300
  [...]
  [<fffffc0001b0d834>] mc146818_set_time+0x74/0x450
  [<fffffc0001f090cc>] _raw_spin_lock_irqsave+0x7c/0xc0
  [<fffffc0001042f90>] do_remote_set+0x90/0xc0
  [<fffffc000119c1a4>] __flush_smp_call_function_queue+0x314/0x5c0
  [<fffffc000119c474>] generic_smp_call_function_single_interrupt+0x24/0x40
  [<fffffc000103d984>] handle_ipi+0xa4/0x230
  [<fffffc0001037044>] do_entInt+0x1a4/0x2e0

The rtc class ops are always called from process context, so there is no
reason to run the access from an interrupt at all.  Use work_on_cpu() to
run it in a worker on the boot cpu.  Alpha does not support cpu hotplug,
so the boot cpu cannot go offline while the work is pending.

Tested on an AlphaServer ES47 (Marvel/EV7): hwclock read and write
pinned to a non-boot cpu, twenty times, with no splat.

Signed-off-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Magnus Lindholm <linmag7@gmail.com>
Link: https://lore.kernel.org/r/20260810202835.3592833-1-mattst88@gmail.com
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>

arch/alpha/kernel/rtc.c

@@ -15,6 +15,7 @@
 #include <linux/bcd.h>
 #include <linux/rtc.h>
 #include <linux/platform_device.h>
+#include <linux/workqueue.h>
 
 #include "proto.h"
 
@@ -142,54 +143,40 @@ static const struct rtc_class_ops alpha_rtc_ops = {
 };
 
 /*
- * Similarly, except do the actual CMOS access on the boot cpu only.
- * This requires marshalling the data across an interprocessor call.
+ * Similarly, except do the actual CMOS access on the boot cpu only.  The
+ * access polls for the RTC update cycle and takes rtc_lock, so run it in a
+ * worker on that cpu rather than from an interprocessor interrupt.
  */
 
 #if defined(CONFIG_SMP) && \
     (defined(CONFIG_ALPHA_GENERIC) || defined(CONFIG_ALPHA_MARVEL))
 # define HAVE_REMOTE_RTC 1
 
-union remote_data {
-	struct rtc_time *tm;
-	long retval;
-};
-
-static void
+static long
 do_remote_read(void *data)
 {
-	union remote_data *x = data;
-	x->retval = alpha_rtc_read_time(NULL, x->tm);
+	return alpha_rtc_read_time(NULL, data);
 }
 
 static int
 remote_read_time(struct device *dev, struct rtc_time *tm)
 {
-	union remote_data x;
-	if (smp_processor_id() != boot_cpuid) {
-		x.tm = tm;
-		smp_call_function_single(boot_cpuid, do_remote_read, &x, 1);
-		return x.retval;
-	}
+	if (smp_processor_id() != boot_cpuid)
+		return work_on_cpu(boot_cpuid, do_remote_read, tm);
 	return alpha_rtc_read_time(NULL, tm);
 }
 
-static void
+static long
 do_remote_set(void *data)
 {
-	union remote_data *x = data;
-	x->retval = alpha_rtc_set_time(NULL, x->tm);
+	return alpha_rtc_set_time(NULL, data);
 }
 
 static int
 remote_set_time(struct device *dev, struct rtc_time *tm)
 {
-	union remote_data x;
-	if (smp_processor_id() != boot_cpuid) {
-		x.tm = tm;
-		smp_call_function_single(boot_cpuid, do_remote_set, &x, 1);
-		return x.retval;
-	}
+	if (smp_processor_id() != boot_cpuid)
+		return work_on_cpu(boot_cpuid, do_remote_set, tm);
 	return alpha_rtc_set_time(NULL, tm);
 }