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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>
This commit is contained in:
Tejun Heo 2010-06-03 11:35:03 +02:00 committed by Jeff Garzik
parent 386f40c86d
commit b475a3b83a
1 changed files with 16 additions and 2 deletions
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18
drivers/ata/sata_via.c
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@ -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);