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>

diff --git a/drivers/ata/ahci.c b/drivers/ata/ahci.c
index 061817a3a0e5a1ce639cb24e0716c425d2caebb5..5e6468a7ca4bd2a051b6ee9244297933a3ee7a60 100644 (file)
--- 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);