On 09/13/2012 04:56 PM, James Bottomley wrote:

On Thu, 2012-09-13 at 16:49 +0800, Aaron Lu wrote:

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On 09/13/2012 04:37 PM, James Bottomley wrote:

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On Thu, 2012-09-13 at 16:23 +0800, Aaron Lu wrote:

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On 09/13/2012 04:14 PM, James Bottomley wrote:

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On Thu, 2012-09-13 at 15:40 +0800, Aaron Lu wrote:

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The ready_to_power_off flag is used to give indication to ATA layer
if this device's power can be removed when runtime suspended.

This flag is determined by individual SCSI driver like sr, sd.

This flag is introduced to support zero power ODD. When ODD
is runtime suspended, it may not be OK to remove its power.

But for disk, it is always OK to be powered off, so set this flag.

It is? I may have missed this, but where do you flush the cache of write
back cache devices you're about to power off?

I suppose that is handled in sd_suspend callback, the power off happens
after a device is runtime suspended.

Well that would mean something is wrong somewhere:  For runtime power
management using idle timers and forced standby, there's no need to

The current mechanism for scsi disk runtime pm is based on open/close.
If there is some process opened this block device, it will be in active
state; only when all opened session exited, it will enter runtime
suspend state.

A mounted disk is open for the period of the mount.  I thought the use
case for runtime PM was the laptop one but most laptops have a single
device to use as root, so if you never use runtime PM on an open device,
you never use it on 99% of our target systems ... doesn't that make the
feature a bit useless?

I agree, but it may be helpful in some cases.

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flush the cache (if the drive goes into standby on its own as a result
of an idle timeout, the cache will never flush).  The cache needs to
flush before we power off the device: that's before the system goes into
S3, or now before you power it off at runtime.  Flushing the cache on
runtime transitions to standby will likely cause performance problems
since that happens quite often.

As explained above, it didn't happen that often, especially for user who
has only one disk, the disk will be mounted, which makes it never be
able to enter runtime suspend state.

So what's the target audience for the feature.  If it isn't laptops or
standard desktops, is it the enterprise?

To make this feature useful for normal laptop user, a better mechanism
for scsi disk runtime pm is needed. Alan Stern and Lin Ming has been
working on this, and I'll see if I can make that patch work later.

So I think this is basically 2 things, one is the runtime suspend of the
disk, another is when it is runtime suspended, how to remove its power.
I'm currently doing the latter one, which is simpler, so I want to do it
first :-)

And there may exist some cases this can be helpful, if user has 2 or
more disks attached and he is only using one of them or some other
corner cases that I don't know.

Considering the effort to implement this feature pretty small, and it
shouldn't cause trouble for existing system, I think this may be worth
it.

Thanks,
Aaron