cxgb4: For T4, don't read the Firmware Mailbox Control register

T4 doesn't have the Shadow copy of the register which we can read without
side effect. So don't read mbox control register for T4 adapter
Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/chelsio/cxgb4/cxgb4_debugfs.c

@@ -1128,18 +1128,26 @@ static const struct file_operations devlog_fops = {
 static int mbox_show(struct seq_file *seq, void *v)
 {
 	static const char * const owner[] = { "none", "FW", "driver",
-					      "unknown" };
+					      "unknown", "<unread>" };
 
 	int i;
 	unsigned int mbox = (uintptr_t)seq->private & 7;
 	struct adapter *adap = seq->private - mbox;
 	void __iomem *addr = adap->regs + PF_REG(mbox, CIM_PF_MAILBOX_DATA_A);
-	unsigned int ctrl_reg = (is_t4(adap->params.chip)
-				 ? CIM_PF_MAILBOX_CTRL_A
-				 : CIM_PF_MAILBOX_CTRL_SHADOW_COPY_A);
+
+	/* For T4 we don't have a shadow copy of the Mailbox Control register.
+	 * And since reading that real register causes a side effect of
+	 * granting ownership, we're best of simply not reading it at all.
+	 */
+	if (is_t4(adap->params.chip)) {
+		i = 4; /* index of "<unread>" */
+	} else {
+		unsigned int ctrl_reg = CIM_PF_MAILBOX_CTRL_SHADOW_COPY_A;
 		void __iomem *ctrl = adap->regs + PF_REG(mbox, ctrl_reg);
 
 		i = MBOWNER_G(readl(ctrl));
+	}
+
 	seq_printf(seq, "mailbox owned by %s\n\n", owner[i]);
 
 	for (i = 0; i < MBOX_LEN; i += 8)