On 09/13/2012 04:56 PM, James Bottomley wrote: > On Thu, 2012-09-13 at 16:49 +0800, Aaron Lu wrote: >> On 09/13/2012 04:37 PM, James Bottomley wrote: >>> On Thu, 2012-09-13 at 16:23 +0800, Aaron Lu wrote: >>>> On 09/13/2012 04:14 PM, James Bottomley wrote: >>>>> On Thu, 2012-09-13 at 15:40 +0800, Aaron Lu wrote: >>>>>> 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. > >>> 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 -- 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
