ast-io: Rework ast_sio_is_enabled() test sequence

The postcondition of probing with a lock sequence is easier to make
correct than with unlock. The original implementation left SuperIO
locked after execution which broke an assumption of some callers.

Tested on Garrison, Palmetto without HIOMAP, Palmetto with HIOMAP and
Witherspoon.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

hw/ast-bmc/ast-io.c

@@ -323,20 +323,41 @@ static void ast_setup_sio_irq_polarity(void)
 
 bool ast_sio_is_enabled(void)
 {
+	bool enabled;
 	int64_t rc;
 
-	/* Begin the unlock sequence with a probe to establish presence */
-	rc = lpc_probe_write(OPAL_LPC_IO, 0x2e, 0xa5, 1);
-	if (rc)
-		return false;
+	lock(&bmc_sio_lock);
+	/*
+	 * Probe by attempting to lock the SIO device, this way the
+	 * post-condition is that the SIO device is locked or not able to be
+	 * unlocked. This turns out neater than trying to use the unlock code.
+	 */
+	rc = lpc_probe_write(OPAL_LPC_IO, 0x2e, 0xaa, 1);
+	if (rc) {
+		enabled = false;
+		/* If we can't lock it, then we can't unlock it either */
+		goto out;
+	}
+
+	/*
+	 * Now that we know that is locked and able to be unlocked, unlock it
+	 * if skiboot's recorded device state indicates it was previously
+	 * unlocked.
+	 */
+	if (bmc_sio_cur_dev != BMC_SIO_DEV_NONE) {
+		/* Send SuperIO password */
+		lpc_outb(0xa5, 0x2e);
+		lpc_outb(0xa5, 0x2e);
 
-	/* Complete the unlock sequence if the device is present */
-	lpc_outb(0xa5, 0x2e);
+		/* Ensure the previously selected logical dev is selected */
+		bmc_sio_outb(bmc_sio_cur_dev, 0x07);
+	}
 
-	/* Re-lock to return to a known state */
-	lpc_outb(0xaa, 0x2e);
+	enabled = true;
+out:
+	unlock(&bmc_sio_lock);
 
-	return true;
+	return enabled;
 }
 
 bool ast_sio_init(void)