On 02/17/10 08:10, 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>
..
I totally agree with this patch, but question the timings used to justify it.
Surely the overhead is only 1-2usec for the case where the device
is the one that was already selected (on a "smart" interface) ?
And for the case where the currently selected device is different
than the desired device (the 1msec case), this patch makes little/no difference?
Cheers
---
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;
}
EXPORT_SYMBOL_GPL(ata_sff_dev_select);
@@ -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) {
@@ -1925,6 +1927,8 @@ int ata_sff_prereset(struct ata_link *link, unsigned long deadline)
struct ata_eh_context *ehc =&link->eh_context;
int rc;
+ link->ap->sff_selected = -1; /* Unknown */
+
rc = ata_std_prereset(link, deadline);
if (rc)
return rc;
@@ -2687,7 +2691,7 @@ void ata_bus_reset(struct ata_port *ap)
iowrite8(ap->ctl, ioaddr->ctl_addr);
ap->last_ctl = ap->ctl;
}
-
+ ap->sff_selected = -1;
DPRINTK("EXIT\n");
return;
diff --git a/include/linux/libata.h b/include/linux/libata.h
index 5fb8884..a85adc2 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -725,6 +725,7 @@ struct ata_port {
#ifdef CONFIG_ATA_SFF
struct ata_ioports ioaddr; /* ATA cmd/ctl/dma register blocks */
+ int sff_selected; /* Cache of selected device */
#endif /* CONFIG_ATA_SFF */
u8 ctl; /* cache of ATA control register */
--
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