On Tue, Nov 13, 2018 at 04:11:42PM -0800, Nicolin Chen wrote: > Hi Guenter, > > On Tue, Nov 13, 2018 at 09:21:02AM -0800, Guenter Roeck wrote: > > > > > INA3221 supports both continuous and single-shot modes. When > > > > > running in the continuous mode, it keeps measuring the inputs > > > > > and converting them to the data register even if there are no > > > > > users reading the data out. In this use case, this could be a > > > > > power waste. > > > > > > > > > > So this patch adds a single-shot mode support so that ina3221 > > > > > could do measurement and conversion only if users trigger it, > > > > > depending on the use case where it only needs to poll data in > > > > > a lower frequency. > > > > > > > > > > The change also exposes "mode" and "available_modes" nodes to > > > > > allow users to switch between two operating modes. > > > > > > > > > Lots and lots of complexity for little gain. Sorry, I don't see > > > > the point of this change. > > > > > > The chip is causing considerable power waste on battery-powered > > > devices so we typically use it running in the single-shot mode. > > > > And you need to be able to do that with a sysfs attribute ? > > Are you planning to have some code switching back and forth > > between the modes ? > > > > You'll need to provide a good rationale why this needs to be > > runtime configurable. > > Honestly, our old downstream driver didn't expose it via sysfs. > Instead, it had a built-in "governor" to switch modes based on > the CPU hotplug state and cpufreq. However, the interface used > to register a CPU hotplug notification was already deprecated. > And I don't feel this governor is generic enough to be present > in the mainline code. > > For me, it's not that necessary to be a sysfs attribute. I try > to add it merely because I cannot find a good criteria for the > mode switching in a hwmon driver. So having an open sysfs node > may allow user space power daemon to decide its operating mode, > since it knows which power mode the system is running at: full > speed (charging/charged) or power saving (on-battery), and it > knows how often this exact service will poll the sensor data. > That is bad, because it is not a generic implementation. Userspace would have to account for each individual driver one by one. > An alternative way (without the sysfs node), after looking at > other hwmon code, could be to have a timed polling thread and > read data using an update_interval value from ABI. This might > turn out to be more complicated as it'll also involve settings > of hardware averaging and conversion time. Above all, I cannot > figure out a good threshold of update_interval to switch modes. > update_interval should only be used if it can be configured into hardware, not to trigger a polling thread. It should only be used in the driver to determine caching intervals. > If you can give some advice of a better implementation, that'd > be great. > >From your description, the only real feasible solution would be a generic one, with a well defined interface either to userspace or to the kernel. The best I can think of would be an in-kernel API setting the power operational mode via callbacks. Alternative would be a generic ability to set power operational mode from userspace, using an ABI which applies to all drivers, not just one. I don't know if any of those interfaces exists. If not, this will require a discussion with upstream kernel maintainers, maybe with a strawman proposal (set of patches). We can't just implement a driver specific solution. Guenter
