Tejun Heo wrote:
Hello, Douglas.
Douglas Gilbert wrote:
Tejun,
I note at this point that the IMMED bit in the
START STOP UNIT cdb is clear. [The code might
note that as well.] All SCSI disks that I have
seen, implement the IMMED bit and according to
the SAT standard, so should SAT layers like the
one in libata.
With the IMMED bit clear:
- on spin up, it will wait until disk is ready.
Okay unless there are a lot of disks, in
which case we could ask Matthew Wilcox for help
- on spin down, will wait until media is
stopped. That could be 20 seconds, and if there
were multiple disks ....
I guess the question is do we need to wait until a
disk is spun down before dropping power to it
and suspending.
I think we do. As we're issuing SYNCHRONIZE CACHE prior to spinning
down disks, it's probably okay to drop power early data-integrity-wise
but still...
We can definitely use IMMED=1 during resume (needs to be throttled
somehow tho). This helps even when there is only one disk. We can let
the disk spin up in the background and proceed with the rest of resuming
process. Unfortunately, libata SAT layer doesn't do IMMED and even if
it does (I've tried and have a patch available) it doesn't really work
because during host resume each port enters EH and resets and
revalidates each device. Many if not most ATA harddisks don't respond
to reset or IDENTIFY till it's fully spun up meaning libata EH has to
wait for all drives to spin up. libata EH runs inside SCSI EH thread
meaning SCSI comman issue blocks till libata EH finishes resetting the
port. So, IMMED or not, sd gotta wait for libata disks.
If we want to do parallel spin down, PM core needs to be updated such
that there are two events - issue and done - somewhat similar to what
SCSI is doing to probe devices parallelly. If we're gonna do that, we
maybe can apply the same mechanism to resume path so that we can do
things parallelly IMMED or not.
Seems, there is another way of doing a bank spin up / spin down: doing
it in two passes. On the first pass START_STOP will be issued with
IMMED=1 on all devices, then on the second pass START_STOP will be
issued with IMMED=0. So the devices will spin up / spin down in the
parallel, but synchronously, hence the needed result will be achieved
with minimal code changes, although it will indeed need upper layer
changes in struct device_driver's suspend(), resume(), etc. callers.
Vlad
-
To unsubscribe from this list: send the line "unsubscribe linux-scsi" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
