sata_mv: trial fix for lost NCQ interrupts

[Mark Lord <liml@rtr.ca>]

This patch is for trial/critique use only at the moment.
Once I hear back from a few people who actually use it,
I'll post an updated fix for upstream/backstream inclusion.

I spent this afternoon nitpicking and bitpicking through the interrupt code
in sata_mv.c, and I believe I found a race on the hc_irq_cause register.  The
code was "helpfully" attempting to use read-modify-write to clear individual
port bits there, but this is impossible to do in a race-free fashion.

So.. the obvious fix is to just write the bits being cleared, without touching
anything else.  This will also be faster, too, since no read is required or
desired.  I really don't see a downside, as long as it actually works for everyone.

It does work for me here.

--- linux-2.6.28/drivers/ata/sata_mv.c	2009-01-13 15:57:11.000000000 -0500
+++ linux/drivers/ata/sata_mv.c	2009-01-13 16:03:04.000000000 -0500
@@ -884,18 +884,14 @@
 		int hardport = mv_hardport_from_port(ap->port_no);
 		void __iomem *hc_mmio = mv_hc_base_from_port(
 					mv_host_base(ap->host), hardport);
-		u32 hc_irq_cause, ipending;
+		u32 hc_irq_cause;
 
 		/* clear EDMA event indicators, if any */
 		writelfl(0, port_mmio + EDMA_ERR_IRQ_CAUSE_OFS);
 
-		/* clear EDMA interrupt indicator, if any */
-		hc_irq_cause = readl(hc_mmio + HC_IRQ_CAUSE_OFS);
-		ipending = (DEV_IRQ | DMA_IRQ) << hardport;
-		if (hc_irq_cause & ipending) {
-			writelfl(hc_irq_cause & ~ipending,
-				 hc_mmio + HC_IRQ_CAUSE_OFS);
-		}
+		/* clear EDMA interrupt indicators */
+		hc_irq_cause = (DEV_IRQ | DMA_IRQ) << hardport;
+		writelfl(~hc_irq_cause, hc_mmio + HC_IRQ_CAUSE_OFS);
 
 		mv_edma_cfg(ap, want_ncq);
 
@@ -2821,10 +2817,9 @@
 	/* clear EDMA errors on this port */
 	writel(0, port_mmio + EDMA_ERR_IRQ_CAUSE_OFS);
 
-	/* clear pending irq events */
-	hc_irq_cause = readl(hc_mmio + HC_IRQ_CAUSE_OFS);
-	hc_irq_cause &= ~((DEV_IRQ | DMA_IRQ) << hardport);
-	writelfl(hc_irq_cause, hc_mmio + HC_IRQ_CAUSE_OFS);
+	/* clear EDMA interrupt indicators */
+	hc_irq_cause = (DEV_IRQ | DMA_IRQ) << hardport;
+	writelfl(~hc_irq_cause, hc_mmio + HC_IRQ_CAUSE_OFS);
 
 	mv_enable_port_irqs(ap, ERR_IRQ);
 }