Re: [RFC PATCH] usb/acpi: Add support usb port power off mechanism for device fixed on the motherboard

Greg KH
2012-05-11T18:35:43Z

On Fri, May 11, 2012 at 01:44:26PM -0400, Alan Stern wrote:
> On Sat, 12 May 2012, Lan Tianyu wrote:
>
> > The power saving depends on devices. I test a usb3.0 ssd. The power saving of
> > power off is about 2.2w more than just selective suspend. In theory, power
> > off can help to save remaining power after selective suspend.
>
> That's a lot of power! Suspended USB devices aren't supposed to
> consume more than 2.5 mA of bus current, which at 5 V amounts to <=
> 0.0125 W. Does the port really use that much? Or does the SSD have a
> separate power supply that it disables when port power is removed?
>
> > > The patch did not address the case of powering down ports that have no
> > > devices attached. That might be a better place to start, because it's
> > > simpler, even though it might not yield as much power savings.
> >
> > Do you mean internal ports?
>
> Internal or external.
>
> > From my opinion, ACPI will tell us whether the port is connectable or not.
>
> ACPI will tell you about some ports, not others (for example, ACPI
> knows nothing about the ports on hubs that the user plugs in). On
> other systems, a Device Tree database might provide the same
> information.
>
> > When the internal port is not connectable, this means the port is not used
> > and this patch will power down the port.
>
> ...
>
> > For external ports, this should be associated with sys file control. The users
> > need to determine when they should be power off.
>
> You don't mean "external", you mean "not described as unconnectable by
> ACPI".
>
> > So I should work on the external ports without devices firstly and
> > add the sys file for user to control?
>
> Yes, I think so. It will be less controversial and probably simpler.
> When that mechanism is ready, you should be able to use it
> automatically for unconnectable ports.
>
> One tricky thing: In theory, there should be a separate sysfs file for
> each port. That seems like a lot of overhead though; is there any way
> to present the information in a single file that won't offend sysfs
> purists?

Why is that a lot of "overhead"? It's what, 7-9 files max? As Sarah
points out, one file for all ports is racy and can get to be a mess.

But then again, I'm a "sysfs purist" :)

greg k-h
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