libata: add comment documenting PIO latency issues on UP

Based on:
http://lkml.indiana.edu/hypermail/linux/kernel/0908.2/01420.html

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index dc72690ed5db..22ff51bdbc8a 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -6616,6 +6616,13 @@ static int __init ata_init(void)
 {
         ata_parse_force_param();
 
+        /*
+         * FIXME: In UP case, there is only one workqueue thread and if you
+         * have more than one PIO device, latency is bloody awful, with
+         * occasional multi-second "hiccups" as one PIO device waits for
+         * another.  It's an ugly wart that users DO occasionally complain
+         * about; luckily most users have at most one PIO polled device.
+         */
         ata_wq = create_workqueue("ata");
         if (!ata_wq)
                 goto free_force_tbl;