[PATCH] AHCI: fix Null pointer dereference in achi_host_active()

[Xiaotian Feng <xtfeng@gmail.com>]

commit b29900e6 introuded a regression, which resulted Null pointer
dereference for achi host with dummy ports. For ahci ports, when the
port is dummy port, its private_data will be NULL, as ata_dummy_port_ops
doesn't support ->port_start.

Reported-and-tested-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Xiaotian Feng <xtfeng@gmail.com>
Cc: Alexander Gordeev <agordeev@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: linux-ide@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 drivers/ata/ahci.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/ata/ahci.c b/drivers/ata/ahci.c
index 5064f3e..f1de689 100644
--- a/drivers/ata/ahci.c
+++ b/drivers/ata/ahci.c
@@ -1147,10 +1147,16 @@ int ahci_host_activate(struct ata_host *host, int irq, unsigned int n_msis)
 
 	for (i = 0; i < host->n_ports; i++) {
 		struct ahci_port_priv *pp = host->ports[i]->private_data;
+		const char *desc;
 
+		if (ata_port_is_dummy(host->ports[i]))
+			desc = dev_driver_string(host->dev);
+		else
+			desc = pp->irq_desc;
+ 
 		rc = devm_request_threaded_irq(host->dev,
 			irq + i, ahci_hw_interrupt, ahci_thread_fn, IRQF_SHARED,
-			pp->irq_desc, host->ports[i]);
+			desc, host->ports[i]);
 		if (rc)
 			goto out_free_irqs;
 	}
-- 
1.7.9.6 (Apple Git-31.1)