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Subject[rfc/patch] libata -- port configurable delays
Hello Jeff et al,

The patch below makes the delays in ata_pause() and ata_busy_wait()
configurable on a per-port basis, and enables the no delay flag on
the one chipset I've tested on. Getting rid of the delays is worth
quite a bit: doing sequential 512 byte O_DIRECT AIO reads results in
a drop from 35.743s to 29.205s using simple-aio-min_nr 20480 10 (a copy
is available at http://www.kvack.org/~bcrl/simple-aio-min_nr.c).
Before this patch __delay() is the number one entry in oprofile
results for this workload. Does this look like a reasonable approach
for chipsets that aren't completely braindead? Cheers,

-ben
--
"Time is what keeps everything from happening all at once." -- John Wheeler

CPU: P4 / Xeon with 2 hyper-threads, speed 2994.52 MHz (estimated)
Counted INSTR_RETIRED events (retired instructions) with a unit mask of 0x03 (mu
ltiple flags) count 90000
samples % app name symbol name
6676 19.5011 vmlinux __delay
923 2.6962 vmlinux __blockdev_direct_IO
733 2.1411 vmlinux memset
711 2.0769 vmlinux kmem_cache_alloc
698 2.0389 vmlinux kmem_cache_free
639 1.8666 vmlinux mempool_alloc
455 1.3291 vmlinux as_insert_request
440 1.2853 vmlinux scsi_request_fn
404 1.1801 vmlinux ext3_get_branch
404 1.1801 vmlinux mempool_free
374 1.0925 vmlinux scsi_dispatch_cmd
356 1.0399 vmlinux __mod_timer
352 1.0282 vmlinux __make_request
...

Signed-off-by: Benjamin LaHaise <benjamin.c.lahaise@intel.com>
diff -purN v2.6.12-rc4/drivers/scsi/ata_piix.c libata-rc4/drivers/scsi/ata_piix.c
--- v2.6.12-rc4/drivers/scsi/ata_piix.c 2005-04-28 11:01:54.000000000 -0400
+++ libata-rc4/drivers/scsi/ata_piix.c 2005-05-13 13:30:39.000000000 -0400
@@ -228,6 +228,7 @@ static struct ata_port_info piix_port_in
.sht = &piix_sht,
.host_flags = ATA_FLAG_SATA | ATA_FLAG_SRST |
PIIX_FLAG_COMBINED | PIIX_FLAG_CHECKINTR |
+ ATA_FLAG_NO_UDELAY |
ATA_FLAG_SLAVE_POSS,
.pio_mask = 0x1f, /* pio0-4 */
.mwdma_mask = 0x07, /* mwdma0-2 */
diff -purN v2.6.12-rc4/include/linux/libata.h libata-rc4/include/linux/libata.h
--- v2.6.12-rc4/include/linux/libata.h 2005-04-06 17:28:10.000000000 -0400
+++ libata-rc4/include/linux/libata.h 2005-05-13 13:32:15.000000000 -0400
@@ -113,6 +113,7 @@ enum {
ATA_FLAG_MMIO = (1 << 6), /* use MMIO, not PIO */
ATA_FLAG_SATA_RESET = (1 << 7), /* use COMRESET */
ATA_FLAG_PIO_DMA = (1 << 8), /* PIO cmds via DMA */
+ ATA_FLAG_NO_UDELAY = (1 << 9), /* don't udelay on port access */

ATA_QCFLAG_ACTIVE = (1 << 1), /* cmd not yet ack'd to scsi lyer */
ATA_QCFLAG_SG = (1 << 3), /* have s/g table? */
@@ -469,7 +470,8 @@ static inline u8 ata_chk_status(struct a
static inline void ata_pause(struct ata_port *ap)
{
ata_altstatus(ap);
- ndelay(400);
+ if (!(ap->flags & ATA_FLAG_NO_UDELAY))
+ ndelay(400);
}

static inline u8 ata_busy_wait(struct ata_port *ap, unsigned int bits,
@@ -478,7 +480,8 @@ static inline u8 ata_busy_wait(struct at
u8 status;

do {
- udelay(10);
+ if (!(ap->flags & ATA_FLAG_NO_UDELAY))
+ udelay(10);
status = ata_chk_status(ap);
max--;
} while ((status & bits) && (max > 0));
-
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