rtc: cmos: avoid taking rtc_lock for extended period of time

On my device reading entirety of /sys/devices/pnp0/00:03/cmos_nvram0/nvmem
takes about 9 msec during which time interrupts are off on the CPU that
does the read and the thread that performs the read can not be migrated
or preempted by another higher priority thread (RT or not).

Allow readers and writers be preempted by taking and releasing rtc_lock
spinlock for each individual byte read or written rather than once per
read/write request.

Signed-off-by: Dmitry Torokhov <[e-mail]>
Reviewed-by: Mateusz Jończyk <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/rtc-cmos.c

···
 	unsigned char *buf = val;
 
 	off += NVRAM_OFFSET;
-	spin_lock_irq(&rtc_lock);
-	for (; count; count--, off++) {
+	for (; count; count--, off++, buf++) {
+		guard(spinlock_irq)(&rtc_lock);
 		if (off < 128)
-			*buf++ = CMOS_READ(off);
+			*buf = CMOS_READ(off);
 		else if (can_bank2)
-			*buf++ = cmos_read_bank2(off);
+			*buf = cmos_read_bank2(off);
 		else
-			break;
+			return -EIO;
 	}
-	spin_unlock_irq(&rtc_lock);
 
-	return count ? -EIO : 0;
+	return 0;
 }
 
 static int cmos_nvram_write(void *priv, unsigned int off, void *val,
···
 	 * NVRAM to update, updating checksums is also part of its job.
 	 */
 	off += NVRAM_OFFSET;
-	spin_lock_irq(&rtc_lock);
-	for (; count; count--, off++) {
+	for (; count; count--, off++, buf++) {
 		/* don't trash RTC registers */
 		if (off == cmos->day_alrm
 				|| off == cmos->mon_alrm
 				|| off == cmos->century)
-			buf++;
-		else if (off < 128)
-			CMOS_WRITE(*buf++, off);
-		else if (can_bank2)
-			cmos_write_bank2(*buf++, off);
-		else
-			break;
-	}
-	spin_unlock_irq(&rtc_lock);
+			continue;
 
-	return count ? -EIO : 0;
+		guard(spinlock_irq)(&rtc_lock);
+		if (off < 128)
+			CMOS_WRITE(*buf, off);
+		else if (can_bank2)
+			cmos_write_bank2(*buf, off);
+		else
+			return -EIO;
+	}
+
+	return 0;
 }
 
 /*----------------------------------------------------------------*/