sata_via: explain the magic fix

Add Joseph Chan's explanation of the problem and workaround to the
VT6421 magic fix.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Joseph Chan <JosephChan@via.com.tw>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

1 files changed, 16 insertions, 2 deletions

diff --git a/drivers/ata/sata_via.c b/drivers/ata/sata_via.c
index 0ecd0f6aa2c0..4730c42a5ee5 100644
--- a/drivers/ata/sata_via.c
+++ b/drivers/ata/sata_via.c
@@ -578,10 +578,24 @@ static void svia_configure(struct pci_dev *pdev)
 
         /*
          * vt6421 has problems talking to some drives.  The following
-         * is the magic fix from Joseph Chan <JosephChan@via.com.tw>.
-         * Please add proper documentation if possible.
+         * is the fix from Joseph Chan <JosephChan@via.com.tw>.
+         *
+         * When host issues HOLD, device may send up to 20DW of data
+         * before acknowledging it with HOLDA and the host should be
+         * able to buffer them in FIFO.  Unfortunately, some WD drives
+         * send upto 40DW before acknowledging HOLD and, in the
+         * default configuration, this ends up overflowing vt6421's
+         * FIFO, making the controller abort the transaction with
+         * R_ERR.
+         *
+         * Rx52[2] is the internal 128DW FIFO Flow control watermark
+         * adjusting mechanism enable bit and the default value 0
+         * means host will issue HOLD to device when the left FIFO
+         * size goes below 32DW.  Setting it to 1 makes the watermark
+         * 64DW.
          *
          * https://bugzilla.kernel.org/show_bug.cgi?id=15173
+         * http://article.gmane.org/gmane.linux.ide/46352
          */
         if (pdev->device == 0x3249) {
                 pci_read_config_byte(pdev, 0x52, &tmp8);