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>
diff --git a/drivers/ata/sata_via.c b/drivers/ata/sata_via.c
index 0ecd0f6..4730c42 100644
--- a/drivers/ata/sata_via.c
+++ b/drivers/ata/sata_via.c
@@ -578,10 +578,24 @@
/*
* 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);