Re: [RFC][PATCH 2/3] PM / sleep: Mechanism to avoid resuming runtime-suspended devices unnecessarily

[Rafael J. Wysocki]

On Friday, May 16, 2014 08:20:55 AM Jacob Pan wrote:
> On Thu, 15 May 2014 11:58:55 -0400 (EDT)
> Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> 
> > On Thu, 15 May 2014, Jacob Pan wrote:
> > 
> > > On Thu, 15 May 2014 10:29:42 -0400 (EDT)
> > > Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> > > 
> > > > On Thu, 15 May 2014, Jacob Pan wrote:
> > > > 
> > > > > > > should we respect ignore_children flag here? not all parent
> > > > > > > devices create children with proper .prepare() function.
> > > > > > > this allows parents override children.
> > > > > > > I am looking at USB, a USB device could have logical
> > > > > > > children such as ep_xx, they don't go through the same
> > > > > > > subsystem .prepare().
> > > > > > 
> > > > > > Well, I'm not sure about that.  Let me consider that for a
> > > > > > while.
> > > > > OK. let me be more clear about the situation i see in USB.
> > > > > Correct me if I am wrong, a USB device will always has at least
> > > > > one endpoint/ep_00 as a kid for control pipe, it is a logical
> > > > > device. So when device_prepare() is called, its call back is
> > > > > NULL which makes .direct_complete = 0. Since children device
> > > > > suspend is called before parents, the parents .direct_complete
> > > > > flag will always get cleared.
> > > > > 
> > > > > What i am trying to achieve here is to see if we avoid resuming
> > > > > built-in (hardwired connect_type) non-hub USB devices based on
> > > > > this new patchset. E.g. we don't want to resume/suspend USB
> > > > > camera every time in system suspend/resume cycle if they are
> > > > > already rpm suspended. We can save ~100ms resume time for the
> > > > > devices we have tested.
> > > > 
> > > > This is a good point, but I don't think it is at all related to 
> > > > ignore_children.
> > > > 
> > > > Instead, it seems that the best way to solve it would be to add a 
> > > > ->prepare() handler for usb_ep_device_type that would always turn 
> > > > on direct_complete.
> > > > 
> > > yeah, that would solve the problem with EP device type. But what
> > > about other subdevices. e.g. for USB camera, uvcvideo device? We
> > > can add .prepare(return 1;) for each level but would it be better
> > > to have a flag similar to ignore_children if not ignore_children
> > > itself.
> > 
> > Something like that could always be added.
> or, how about if a device's .prepare() is NULL, we could
> assume .direct_resume() should be set. i.e.

You mean direct_complete (which is a flag, not a function), I suppose?

Wouldn't that go a bit too far?  It seems to be based on the assumption that
all devices having no ->prepare() callback can be safely left in runtime
suspend over a system suspend/resume cycle, but is that assumption actually
satisfied for all such devices?

> --- a/drivers/base/power/main.c
> +++ b/drivers/base/power/main.c
> @@ -1539,7 +1539,7 @@ static int device_prepare(struct device *dev,
> pm_message_t state) pm_runtime_put(dev);
>                 return ret;
>         }
> -       dev->power.direct_complete = ret > 0 && state.event ==
> PM_EVENT_SUSPEND
> +       dev->power.direct_complete = (!callback || ret > 0) &&
> state.event == PM_EVENT_SUSPEND && pm_runtime_suspended(dev);
>         dev_dbg(dev, "%s:direct_complete %d, info %s\n", __func__,
> dev->power.direct_complete, info);
> 
> > 
> > > Actually, I don't understand why this is not related to
> > > ignore_children. Could you explain?
> > 
> > It's hard to explain why two things are totally separate.  Much
> > better for you to describe why you think they _are_ related, so that
> > I can explain how you are wrong.
> > 
> > > If the parent knows it can ignore children and already rpm
> > > suspended, why do we still ask children?
> > 
> > The "ignore_children" flag doesn't mean that the parent can ignore
> > its children.  It means that the PM core is allowed to do a runtime
> > suspend of the parent while leaving the children at full power.
> > 
> > In particular, it doesn't mean that the children's ->suspend()
> > callback will work correctly if it is called while the parent is
> > runtime suspended.
> that explains my question about ignore_chilren flag. thanks.
> > 
> > Alan Stern
> > 
> 
> [Jacob Pan]
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-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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