On Sun, 18 Nov 2012, Tejun Heo wrote: > Hello, Alan. Hi. > On Mon, Nov 12, 2012 at 02:34:01PM -0500, Alan Stern wrote: > > It wouldn't do anything for the device, but it would allow the device's > > parent to go to low power in between polls. That's why Aaron at one > > point suggested lengthening the polling interval. > > Ah, okay. Entering and leaving suspend even every two seconds may > save noticeable power for some controllers. Does that, tho, mean that > the port would get reset every two seconds? I don't know how suspend works for (S)ATA ports. For USB mass storage, suspend/resume does not involve any resets unless something goes wrong. > If so (I can't see how > else it would behave), it may not really work. I mean, you would be > spending about ten seconds reinitiaizing everything and then enter > sleep and do it all over again two seconds later, most likely wasting > power in the process. Sounds weird to me. What am I missing? Does it really take 10 seconds to recover from an ATA suspend? That sounds awfully long. The two second value is merely the default interval for media polling. Devices with no async notification mechanism require some sort of polling if anybody wants to know when media is inserted or removed, and the polling can't be carried out if the device's port is suspended. You asked about autopm. "Autopm" is a rather vague and ugly term I made up; it means that the kernel automatically suspends the device after some specified idle time. In the case of optical drives, "idle" currently means the device file is not open (which implies the drive doesn't contain a mounted filesystem). As far as I know, the kernel has no direct way to affect the power level of an optical drive that isn't ZP. For these devices "suspended" is merely a logical state, meaning that the device isn't in use and therefore its parent can be put into a low-power mode. Regardless, an optical drive can't remain suspended when polling occurs. So unless there's some other mechanism for getting the information about media changes, suspended drives (and their parents) will have to be resumed periodically for polling. Hence there's a tradeoff. How can we use the minimum amount of energy while still polling the drive acceptably often? In general, the kernel doesn't know. That's why these things can be controlled from userspace. And the answer may be different for ATA drives vs. USB-connected drives. Does this answer your questions? Alan Stern -- 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
