David:
I did some testing with "echo 4 >/proc/acpi/sleep" and looked at the code
in drivers/base/power. What a mess! The routines there have no real
concept of suspending or resuming individual devices, only the entire
system. This business of handling separately drivers that require
interrupts to be disabled looks ridiculous -- and it will break the
necessary topological ordering of the device lists.
I also found a few bugs in the USB code. The problems can be summed up
like this: Who is responsible, the caller or the callee, for checking that
old_state != new_state (i.e., whether the call is necessary), and who is
responsible for setting dev->power.power_state to new_state? The code is
inconsistent in its answers to these questions, leading to various weird
behaviors.
For instance, interface drivers' resume() method can be called either from
finish_port_resume() or from usb_generic_resume(). One checks whether the
interface has already been resumed and the other doesn't. One of the
callees, hub_resume(), doesn't check. Hence there can be multiple resumes
of a hub.
As another example, the bus->op->hub_suspend() call in
__usb_suspend_device() doesn't have to set power_state because the code
following the call does so. But the corresponding call to
bus->op->hub_resume() in usb_resume_device() does have to set power_state.
Even the suspend_device() and resume_device() routines in
drivers/base/power/{suspend,resume}.c are inconsistent about checking
whether the device is already suspended/resumed. At least they are
consistent in relying on the callee to set power_state.
Presumably the callee is always supposed to set power_state, rather than
the caller -- that is the majority behavior. But who's supposed to weed
out unnecessary calls?
Alan Stern
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