sata_sil: ignore and clear spurious IRQs while executing commands by polling

sata_sil used to trigger HSM error if IRQ occurs during polling
command.  This didn't matter because polling wasn't used in sata_sil.
However, as of 2.6.20, all IDENTIFYs are performed by polling and
device detection sometimes fails due to spurious IRQ.  This patch
makes sata_sil ignore and clear spurious IRQ while executing commands
by polling.

This fixes bug#7996 and IMHO should also be included in -stable.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/ata/sata_sil.c

@@ -383,9 +383,15 @@ static void sil_host_intr(struct ata_port *ap, u32 bmdma2)
 		goto freeze;
 	}
 
-	if (unlikely(!qc || qc->tf.ctl & ATA_NIEN))
+	if (unlikely(!qc))
 		goto freeze;
 
+	if (unlikely(qc->tf.flags & ATA_TFLAG_POLLING)) {
+		/* this sometimes happens, just clear IRQ */
+		ata_chk_status(ap);
+		return;
+	}
+
 	/* Check whether we are expecting interrupt in this state */
 	switch (ap->hsm_task_state) {
 	case HSM_ST_FIRST: