Dump stack once on unsupported commands to see who is submitting them.
(#632753)

--- linux-2.6.34.noarch.orig/drivers/ata/pata_it821x.c
+++ linux-2.6.34.noarch/drivers/ata/pata_it821x.c
@@ -399,6 +399,16 @@ static void it821x_passthru_dev_select(s
 	ata_sff_dev_select(ap, device);
 }
 
+static void it821x_dump_stack_once(void)
+{
+	static int dumped = 0;
+
+	if (!dumped) {
+		dump_stack();
+		dumped = 1;
+	}
+}
+
 /**
  *	it821x_smart_qc_issue		-	wrap qc issue prot
  *	@qc: command
@@ -433,6 +443,7 @@ static unsigned int it821x_smart_qc_issu
 			return ata_sff_qc_issue(qc);
 	}
 	printk(KERN_DEBUG "it821x: can't process command 0x%02X\n", qc->tf.command);
+	it821x_dump_stack_once();
 	return AC_ERR_DEV;
 }