libata: retry link resume if necessary

Interestingly, when SIDPR is used in ata_piix, writes to DET in
SControl sometimes get ignored leading to detection failure.  Update
sata_link_resume() such that it reads back SControl after clearing DET
and retry if it's not clear.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: fengxiangjun <fengxiangjun@neusoft.com>
Reported-by: Jim Faulkner <jfaulkne@ccs.neu.edu>
Cc: stable@kernel.org
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/ata/libata-core.c

@@ -3790,22 +3790,46 @@ int sata_link_debounce(struct ata_link *link, const unsigned long *params,
 int sata_link_resume(struct ata_link *link, const unsigned long *params,
 		     unsigned long deadline)
 {
+	int tries = ATA_LINK_RESUME_TRIES;
 	u32 scontrol, serror;
 	int rc;
 
 	if ((rc = sata_scr_read(link, SCR_CONTROL, &scontrol)))
 		return rc;
 
+	/*
+	 * Writes to SControl sometimes get ignored under certain
+	 * controllers (ata_piix SIDPR).  Make sure DET actually is
+	 * cleared.
+	 */
+	do {
 		scontrol = (scontrol & 0x0f0) | 0x300;
-
 		if ((rc = sata_scr_write(link, SCR_CONTROL, scontrol)))
 			return rc;
-
-	/* Some PHYs react badly if SStatus is pounded immediately
-	 * after resuming.  Delay 200ms before debouncing.
+		/*
+		 * Some PHYs react badly if SStatus is pounded
+		 * immediately after resuming.  Delay 200ms before
+		 * debouncing.
 		 */
 		msleep(200);
 
+		/* is SControl restored correctly? */
+		if ((rc = sata_scr_read(link, SCR_CONTROL, &scontrol)))
+			return rc;
+	} while ((scontrol & 0xf0f) != 0x300 && --tries);
+
+	if ((scontrol & 0xf0f) != 0x300) {
+		ata_link_printk(link, KERN_ERR,
+				"failed to resume link (SControl %X)\n",
+				scontrol);
+		return 0;
+	}
+
+	if (tries < ATA_LINK_RESUME_TRIES)
+		ata_link_printk(link, KERN_WARNING,
+				"link resume succeeded after %d retries\n",
+				ATA_LINK_RESUME_TRIES - tries);
+
 	if ((rc = sata_link_debounce(link, params, deadline)))
 		return rc;
 

include/linux/libata.h

@@ -354,6 +354,9 @@ enum {
 	/* max tries if error condition is still set after ->error_handler */
 	ATA_EH_MAX_TRIES	= 5,
 
+	/* sometimes resuming a link requires several retries */
+	ATA_LINK_RESUME_TRIES	= 5,
+
 	/* how hard are we gonna try to probe/recover devices */
 	ATA_PROBE_MAX_TRIES	= 3,
 	ATA_EH_DEV_TRIES	= 3,