On Wed, Oct 10, 2018 at 05:24:12PM -0700, Nicolin Chen wrote: > Hi Guenter, > > On Wed, Oct 10, 2018 at 04:43:00PM -0700, Guenter Roeck wrote: > > > > The effort to do all this using CPU cycles would in most if not all cases > > > > outweigh any perceived power savings. As such, I just don't see the > > > > practical use case. > > > > > > It really depends on the use case and how often the one-shot gets > > > triggered. For battery-powered devices, running in the continuous > > > mode does consume considerable power based on the measurement from > > > our power folks. If a system is running in a power sensitive mode, > > > while it still needs to occasionally check the inputs, it could be > > > a use case for one-shot mode, though it's purely a user decision. > > > > > That would actually be a use case for runtime power management. > > The power used by a modern sensor chip is miniscule compared > > to the power consumed by other components in the system, > > which may explain why no one bothered looking into runtime > > power management for sensors. > > > > This is also an argument against any device or subsystem specific solution: > > Users will want to be able to control power consumption for all devices > > in the system, not just for sensors. A device specific power control > > mechanism would, from user space perspective, be a nightmare. > > That makes sense to me. Actually we wanted a solution more on the > driver side so that user space doesn't need to be involved. But our > downstream solution (switching different modes when certain number > of CPUs get hot plugged in/out) wasn't accepted years ago by other > subsystem maintainers, being commented that it's a user decision, > IIRC. So I thought that having a sysfs node might be a generic way > for "user decision". But I guess I should have tried to seek for a > solution in the kernel like runtime PM as you said. > > > > > power-down mode effectively reinvents runtime power management (runtime > > > > suspend/resume support) and is thus simply unacceptable. > > > > > > Similar to one-shot, if a system is in a low power mode where it > > > doesn't want to check the inputs anymore, I feel the user space > > > could at least make the decision to turn on/off the chips, I am > > > not quite sure if the generic runtime PM system already has this > > > kind of support though. > > > > > Please look up "runtime power management". It provides the basic > > mechanism to turn off / disable a device if it is not used. The point > > here is that the basic mechanism is there, even though it may not > > be perfect. If it is not perfect, it needs to be improved. > > Implementing a per-subsystem alternate method would be the wrong > > approach. > > I have implemented runtime PM suspend/resume in other drivers but > what I know is that it relies on someone to call get_sync and put > functions accordingly and it doesn't seem to have built-in system > level low-power or power-sensitive mode for those use cases that > I mentioned previously. But I agree with your point and will take > a closer look. > > One more question here, and this might sound a bit abuse of using > the existing hwmon ABI: would it sound plausible to you that the > driver powers down the chip when all three channels get disabled > via in[123]_enable nodes? :) > I would not call that an abuse, no. Guenter
