Re: [PATCH 1/2] scsi: sd: set ready_to_power_off for scsi disk

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On Thursday 13 September 2012 10:26:44 James Bottomley wrote:
> On Thu, 2012-09-13 at 17:07 +0800, Aaron Lu wrote:

> > 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 :-)
> 
> Well, I don't like the way the interaction of the patches is going.
> You're the one proposing powering down the device outside of the
> standards defined transitions, so you need to be responsible for the
> actions that necessitates, including synchronizing the cache.  The specs
> (SPC-4) say that cache management is explicitly unnecessary for the
> standard SCSI power states (Active, Idle, Standby and Stopped), so
> someone at some point is going to read that and remove the unnecessary
> cache sync in the code.  When that happens, you'll start getting data
> loss.

The cache is handled identically in sd_suspend() and sd_shutdown().
In fact sd_shutdown() will skip handling it if the device has already been
suspended, so the assumption is built into the code and has been so
for a long time.

Though it wouldn't hurt to add a comment that says that the system going
to S3 or S4 will cut power to a lot of disk so that the cache needs to be synced
even if the spec says we need not. Runtime PM doesn't much alter the
situation.

	Regards
		Oliver

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