ahci: give another shot at clearing all bits in irq_stat

Commit ea0c62f7cf70f13a67830471b613337bd0c9a62e tried to clear all
bits in irq_stat but it didn't actually achieve that as irq_stat was
anded with port_map right after read.  This patch makes ahci driver
always use the unmasked value to clear irq_status.

While at it, add explanation on the peculiarities of ahci IRQ
clearing.

This was spotted by Linus Torvalds.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 3 deletions

diff --git a/drivers/ata/ahci.c b/drivers/ata/ahci.c
index 061817a3a0e5..5e6468a7ca4b 100644
--- a/drivers/ata/ahci.c
+++ b/drivers/ata/ahci.c
@@ -1777,7 +1777,7 @@ static irqreturn_t ahci_interrupt(int irq, void *dev_instance)
         struct ahci_host_priv *hpriv;
         unsigned int i, handled = 0;
         void __iomem *mmio;
-        u32 irq_stat;
+        u32 irq_stat, irq_masked;
 
         VPRINTK("ENTER\n");
 
@@ -1786,16 +1786,17 @@ static irqreturn_t ahci_interrupt(int irq, void *dev_instance)
 
         /* sigh.  0xffffffff is a valid return from h/w */
         irq_stat = readl(mmio + HOST_IRQ_STAT);
-        irq_stat &= hpriv->port_map;
         if (!irq_stat)
                 return IRQ_NONE;
 
+        irq_masked = irq_stat & hpriv->port_map;
+
         spin_lock(&host->lock);
 
         for (i = 0; i < host->n_ports; i++) {
                 struct ata_port *ap;
 
-                if (!(irq_stat & (1 << i)))
+                if (!(irq_masked & (1 << i)))
                         continue;
 
                 ap = host->ports[i];
@@ -1812,6 +1813,15 @@ static irqreturn_t ahci_interrupt(int irq, void *dev_instance)
                 handled = 1;
         }
 
+        /* HOST_IRQ_STAT behaves as level triggered latch meaning that
+         * it should be cleared after all the port events are cleared;
+         * otherwise, it will raise a spurious interrupt after each
+         * valid one.  Please read section 10.6.2 of ahci 1.1 for more
+         * information.
+         *
+         * Also, use the unmasked value to clear interrupt as spurious
+         * pending event on a dummy port might cause screaming IRQ.
+         */
         writel(irq_stat, mmio + HOST_IRQ_STAT);
 
         spin_unlock(&host->lock);