On Wednesday, September 14, 2016 02:29:12 AM Rafael J. Wysocki wrote:
> On Wednesday, August 31, 2016 08:15:18 AM Lukas Wunner wrote:
> > Commit 58a1fbbb2ee8 ("PM / PCI / ACPI: Kick devices that might have been
> > reset by firmware") added a runtime resume for devices that were runtime
> > suspended when the system entered sleep.
> >
> > The motivation was that devices might be in a reset-power-on state after
> > waking from system sleep, so their power state as perceived by Linux
> > (stored in pci_dev->current_state) would no longer reflect reality.
> > By resuming such devices, we allow them to return to a low-power state
> > via autosuspend and also bring their current_state in sync with reality.
>
> Not only that.
>
> It allows those devices to respond more timely to user space requests
> occuring right after resume.
>
> Those user space requests often occur sequentially (read from device A,
> write to device B, read from device C, write to device D, for example)
> and without that pre-emptive resume each of them needs to wait for another
> device to (runtime-)resume which results in quite annoying latencies
> observable by users.
>
> > However for devices that are *not* in a reset-power-on state, doing an
> > unconditional resume wastes energy.
>
> It may or may not waste it and this really is a tradeoff.
That said it is a bit inconsistent with the suspend-to-idle case in which
we don't resume those devices, so never mind. :-)
Thanks,
Rafael
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