platform/x86/intel/sdsi: Handle leaky bucket

To prevent an agent from indefinitely holding the mailbox firmware has
implemented a leaky bucket algorithm. Repeated access to the mailbox may
now incur a delay of up to 2.1 seconds. Add a retry loop that tries for
up to 2.5 seconds to acquire the mailbox.

Fixes: 2546c60 ("platform/x86: Add Intel Software Defined Silicon driver")
Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Link: https://lore.kernel.org/r/20220420155622.1763633-2-david.e.box@linux.intel.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

Author: debox1

drivers/platform/x86/intel/sdsi.c

@@ -51,6 +51,8 @@
 #define MBOX_TIMEOUT_US			2000
 #define MBOX_TIMEOUT_ACQUIRE_US		1000
 #define MBOX_POLLING_PERIOD_US		100
+#define MBOX_ACQUIRE_NUM_RETRIES	5
+#define MBOX_ACQUIRE_RETRY_DELAY_MS	500
 #define MBOX_MAX_PACKETS		4
 
 #define MBOX_OWNER_NONE			0x00
@@ -263,7 +265,7 @@ static int sdsi_mbox_acquire(struct sdsi_priv *priv, struct sdsi_mbox_info *info
 {
 	u64 control;
 	u32 owner;
-	int ret;
+	int ret, retries = 0;
 
 	lockdep_assert_held(&priv->mb_lock);
 
@@ -273,13 +275,29 @@ static int sdsi_mbox_acquire(struct sdsi_priv *priv, struct sdsi_mbox_info *info
 	if (owner != MBOX_OWNER_NONE)
 		return -EBUSY;
 
-	/* Write first qword of payload */
-	writeq(info->payload[0], priv->mbox_addr);
+	/*
+	 * If there has been no recent transaction and no one owns the mailbox,
+	 * we should acquire it in under 1ms. However, if we've accessed it
+	 * recently it may take up to 2.1 seconds to acquire it again.
+	 */
+	do {
+		/* Write first qword of payload */
+		writeq(info->payload[0], priv->mbox_addr);
+
+		/* Check for ownership */
+		ret = readq_poll_timeout(priv->control_addr, control,
+			FIELD_GET(CTRL_OWNER, control) == MBOX_OWNER_INBAND,
+			MBOX_POLLING_PERIOD_US, MBOX_TIMEOUT_ACQUIRE_US);
+
+		if (FIELD_GET(CTRL_OWNER, control) == MBOX_OWNER_NONE &&
+		    retries++ < MBOX_ACQUIRE_NUM_RETRIES) {
+			msleep(MBOX_ACQUIRE_RETRY_DELAY_MS);
+			continue;
+		}
 
-	/* Check for ownership */
-	ret = readq_poll_timeout(priv->control_addr, control,
-				 FIELD_GET(CTRL_OWNER, control) & MBOX_OWNER_INBAND,
-				 MBOX_POLLING_PERIOD_US, MBOX_TIMEOUT_ACQUIRE_US);
+		/* Either we got it or someone else did. */
+		break;
+	} while (true);
 
 	return ret;
 }