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

[Aaron Lu <aaron.lu@intel.com>]

On Thu, Sep 13, 2012 at 10:26:44AM +0100, 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

OK, I'll update the code.

> (SPC-4) say that cache management is explicitly unnecessary for the
> standard SCSI power states (Active, Idle, Standby and Stopped), so

Just read the SPC-4 spec, in section 5.12.3, it has words like this:

Logical units that contain cache memory shall write all cached data to
the medium for the logical unit(e.g., as a logical unit would do in
response to a SYNCHRONIZE CACHE command as described SBC-3) prior to
entering into any power condition that prevents accessing the
media(e.g., before a hard drive stops its spindle motor during a change
to the standby power condition).

So this looks like cache needs to be synced before the device enter
standby/stopped power condition. Or do I miss somthing?

> 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.

Indeed, I'll make sure cache gets synced when we are to power off the
device. Thanks for the remind.

-Aaron