On Thursday, January 09, 2014 01:17:12 PM Rafael J. Wysocki wrote: > On Thursday, January 09, 2014 09:29:59 AM Aaron Lu wrote: > > On 01/09/2014 05:21 AM, Alan Stern wrote: > > > On Wed, 8 Jan 2014, Phillip Susi wrote: > > >> You issue a REQUEST SENSE command and that returns status indicating > > >> whether the drive is stopped, or in standby. See my patches. One of > > > > > > I never saw your patches. Where were they posted? > > > > > > If you issue the REQUEST SENSE command in the usual way (going through > > > the SCSI and block layers), and the disk is already in runtime suspend, > > > it won't do what you want. The block layer won't deliver requests > > > until the device leaves the RPM_SUSPENDED state. In addition, when it > > > receives the command the block layer will submit a deferred > > > runtime-resume request, which rather defeats the whole purpose. > > > > > > (I guess you actually saw some of this happen, and that's what led to > > > this email thread in the first place.) > > > > > > It's a knotty situation. The only way to find out whether the disk is > > > spinning is to ask it, which requires doing I/O, which requires > > > spinning up the disk. Maybe we need to add a mechanism to the block > > > layer's runtime PM implementation for addressing just this situation. > > > > I think it's knotty because the runtime PM status is a view from the > > kernel/host side, i.e. it is runtime suspended if it is not being used, > > no matter which power state it is in. The trigger for the PM state > > transition are all based on this, if we want to do it the other way > > around(update device's runtime PM status based on device's actual power > > state), we are in a knotty situation. > > Agreed. That said during system resume, when we are trying to avoid resuming the device to save time/energy, it makes sense to check its physical state and do a pm_request_resume() if that is "on" to avoid a situation in which the device is physically "on", but its runtime PM status is "suspended" and it never gets powered down because of that. Thanks! -- I speak only for myself. Rafael J. Wysocki, Intel Open Source Technology Center. -- 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
