[PATCH] libata: workaround DRQ=1 ERR=1 for ATAPI tape drives

[Albert Lee <albertcc@tw.ibm.com>]

After an error condition, some ATAPI tape drives set DRQ=1 together with ERR=1
when asking the host to transfer the CDB of the next packet command (i.e. request sense).
This patch workarounds the problem by ignoring the ERR bit and proceed REQUEST SENSE.

Signed-off-by: Albert Lee <albertcc@tw.ibm.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Mark Lord <liml@rtr.ca>
---
Tested ok with the Seagate STT8000A tape drive. A similar workaround is seen in ide-tape.c:

	if (status.b.check && pc->c[0] == IDETAPE_REQUEST_SENSE_CMD)
		status.b.check = 0;

Although ide-tape only does it when DRQ=0...

--- linux-2.6.23.1/drivers/ata/libata-core.c.orig	2007-10-13 00:43:44.000000000 +0800
+++ linux-2.6.23.1/drivers/ata/libata-core.c	2007-11-13 17:11:03.000000000 +0800
@@ -4930,11 +4930,18 @@ fsm_start:
 		 * let the EH abort the command or reset the device.
 		 */
 		if (unlikely(status & (ATA_ERR | ATA_DF))) {
-			ata_port_printk(ap, KERN_WARNING, "DRQ=1 with device "
-					"error, dev_stat 0x%X\n", status);
-			qc->err_mask |= AC_ERR_HSM;
-			ap->hsm_task_state = HSM_ST_ERR;
-			goto fsm_start;
+			/* Some ATAPI tape drives forget to clear the ERR bit
+			 * when doing the next command (i.e. request sense). We
+			 * ignore ERR here to workaround and proceed REQUEST_SENSE.
+			 */
+			if (qc->tf.protocol == ATA_PROT_PIO ||
+			    qc->cdb[0] != REQUEST_SENSE) {
+				ata_port_printk(ap, KERN_WARNING, "DRQ=1 with device "
+						"error, dev_stat 0x%X\n", status);
+				qc->err_mask |= AC_ERR_HSM;
+				ap->hsm_task_state = HSM_ST_ERR;
+				goto fsm_start;
+			}
 		}
 
 		/* Send the CDB (atapi) or the first data block (ata pio out).