Re: [RFC 1/4] libata: cache device select

From: Jeff Garzik
Date: Mon Mar 01 2010 - 15:16:01 EST

On 02/17/2010 08:10 AM, Alan Cox wrote:
Avoid the device select overhead on every qc_issue (> 10uS) by caching the
currently selected device. This shows up on profiles under load. Best case
this costs us 10uS for the delay, worst case with a dumb interface it's
costing us about *1mS* a command.

I believe the logic here is sufficient, but would welcome some second reviews
as its not something you want to get wrong !

Signed-off-by: Alan Cox<alan@xxxxxxxxxxxxxxx>

drivers/ata/libata-sff.c | 8 ++++++--
include/linux/libata.h | 1 +
2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/ata/libata-sff.c b/drivers/ata/libata-sff.c
index 63d9c6a..cf0332a 100644
--- a/drivers/ata/libata-sff.c
+++ b/drivers/ata/libata-sff.c
@@ -469,6 +469,7 @@ void ata_sff_dev_select(struct ata_port *ap, unsigned int device)

iowrite8(tmp, ap->ioaddr.device_addr);
ata_sff_pause(ap); /* needed; also flushes, for mmio */
+ ap->sff_selected = device;

@@ -1538,7 +1539,8 @@ unsigned int ata_sff_qc_issue(struct ata_queued_cmd *qc)

/* select the device */
- ata_dev_select(ap, qc->dev->devno, 1, 0);
+ if (qc->dev->devno != ap->sff_selected)
+ ata_dev_select(ap, qc->dev->devno, 1, 0);

/* start the command */
switch (qc->tf.protocol) {

My main worry here is that this logic excises the 150ms wait in ata_dev_select() that has been used effectively to allow ATAPI devices to "collect themselves" after waiting for idle, prior to command issuance.


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