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>

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);