On Tue, 20 Sep 2016, Ulf Hansson wrote:

> >> I am wondering what you think would be a good autosuspend timeout in
> >> this case? It seems to me that the only thing the rtsx driver really
> >> care about is to tell the parent device that it needs to be runtime
> >> resumed during a certain timeframe, more or less it would like to
> >> inherit the parents settings.
> >>
> >> Other mmc hosts, not being usb-mmc devices, are using an autosuspend
> >> timeout of ~50-100ms but that doesn't seem like good value here,
> >> right?
> >
> > Well, if you decide to let the device go into runtime suspend between
> > polls then there's no reason to use autosuspend at all.  Once a poll
> > has ended, you know there won't be any more activity until the next
> > poll.
> We could change that, as currently the approach in the mmc core isn't
> that sophisticated. I even think this has been discussed earlier for
> the very similar reasons regards polling card detect mode.
> I guess the main reason to why we yet have changed this, is because
> mmc host drivers are using an autosuspend timeout of ~50-100 ms, so in
> the end it haven't been such a big deal.

No, it isn't.  Although at a polling interval of 1 second, it means 
reducing the low-power time by as much as 10%.

> > On the other hand, if you decide to keep the device at full power all
> > the time during polling, then any autosuspend timeout larger than 1000
> > ms would do what you want.
> >
> > Mostly I'm concerned about how this will interact with the USB runtime
> > PM.  The thing is, suspending the sdmmc device doesn't save any energy,
> > whereas suspending the USB device does.
> Yes, I agree.
> My concern is also 2s autosuspend timeout which is set for the usb
> device. Somehow I feel we need to be able "share" more information
> between a parent-child relationship, in this case between the sdmmc
> device and the usb device.

I agree, but it's not clear how this should be done.  One easy solution 
would be to turn off USB autosuspend and do all the runtime-PM 
management in the sdmmc and memstick drivers.

> An observation I made, is when the sdmmc device gets runtime resumed
> (pm_runtime_get_sync()), the parent device (the usb device) may also
> become runtime resumed (unless it's already). In this sequence, but
> *only* when actually runtime resuming the usb device, the runtime PM
> core decides to update the last busy mark for the usb device. Should
> it really do that?

Yes, that's deliberate.  The whole idea of autosuspend is to prevent 
the device from being runtime-suspended too soon after it was used, and 
the PM core considers runtime-resume to be a form of usage.

> Moreover, I am curious about the 2s usb timeout. Why isn't that chosen
> to something like ~100ms instead? Is there is a long latency to
> runtime resume the usb device or because we fear to wear out the HW,
> which may be powered on/off too frequently?

When I first implemented runtime PM for the USB stack, I had to choose 
a autosuspend timeout.  Not having any basis for such a choice, and 
figuring that a suspend-resume cycle takes around 100 ms, I decided 
that 2 seconds would be a reasonable value.  But it's just a default; 
drivers and userspace can change it whenever they want.

> If we assume that the usb device shouldn't be used with a timeout less
> than 2s, then I think we have two options:
> *) As the mmc polling timeout is 1s, there is really no point in
> trying to runtime suspend the usb device, it may just be left runtime
> resumed all the time. Wasting power, of course!

Or we can decrease the USB autosuspend delay to 100 ms.

> **) Add an interface to allow dynamically changes of the mmc polling
> timeout to better suit the current user.

Note that the block layer does its own polling for removable media, and
it already has a sysfs interface to control the polling interval (or 
disable it entirely).  But I don't know how the MMC stack interacts 
with the block layer.

One awkward point is that the sdmmc and memstick drivers each do their
own polling.  This is a waste.  You can see it in the usbmon trace;  
every second there are two query-response interactions.  Even if
there's no good way to reduce the number, we should at least try to
synchronize the polls so that the device doesn't need to be resumed
twice every second.

Alan Stern

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