libata: use PIO for non-16 byte aligned ATAPI commands

The IDE driver used DMA for ATAPI commands if READ/WRITE command is
multiple of sector size or sg command is multiple of 16 bytes.  For
libata, READ/WRITE sector alignment is guaranteed by the high level
driver (sr), so we only have to worry about the 16 byte alignment.

This patch makes ata_check_atapi_dma() always request PIO for all data
transfer commands which are not multiple of 16 bytes.

The following reports are related to this problem.

http://bugzilla.kernel.org/show_bug.cgi?id=8605		(confirmed)
http://thread.gmane.org/gmane.linux.kernel/476620	(confirmed)
https://bugzilla.novell.com/show_bug.cgi?id=229260	(probably)

Albert first pointed out the difference between IDE and libata.  Kudos
to him.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Albert Lee <albertcc@tw.ibm.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index 642097a..094b518 100644 (file)
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -4109,6 +4109,7 @@ static void ata_fill_sg(struct ata_queued_cmd *qc)
        if (idx)
                ap->prd[idx - 1].flags_len |= cpu_to_le32(ATA_PRD_EOT);
 }
+
 /**
  *     ata_check_atapi_dma - Check whether ATAPI DMA can be supported
  *     @qc: Metadata associated with taskfile to check
@@ -4126,33 +4127,19 @@ static void ata_fill_sg(struct ata_queued_cmd *qc)
 int ata_check_atapi_dma(struct ata_queued_cmd *qc)
 {
        struct ata_port *ap = qc->ap;
-       int rc = 0; /* Assume ATAPI DMA is OK by default */
-
-       /* some drives can only do ATAPI DMA on read/write */
-       if (unlikely(qc->dev->horkage & ATA_HORKAGE_DMA_RW_ONLY)) {
-               struct scsi_cmnd *cmd = qc->scsicmd;
-               u8 *scsicmd = cmd->cmnd;
-
-               switch (scsicmd[0]) {
-               case READ_10:
-               case WRITE_10:
-               case READ_12:
-               case WRITE_12:
-               case READ_6:
-               case WRITE_6:
-                       /* atapi dma maybe ok */
-                       break;
-               default:
-                       /* turn off atapi dma */
-                       return 1;
-               }
-       }
+
+       /* Don't allow DMA if it isn't multiple of 16 bytes.  Quite a
+        * few ATAPI devices choke on such DMA requests.
+        */
+       if (unlikely(qc->nbytes & 15))
+               return 1;
 
        if (ap->ops->check_atapi_dma)
-               rc = ap->ops->check_atapi_dma(qc);
+               return ap->ops->check_atapi_dma(qc);
 
-       return rc;
+       return 0;
 }
+
 /**
  *     ata_qc_prep - Prepare taskfile for submission
  *     @qc: Metadata associated with taskfile to be prepared