Re: [Qemu-devel] [patch 0/2] USB UHCI global suspend / remote wakeup

[Marcelo Tosatti]

On Fri, Nov 26, 2010 at 12:38:28AM +0000, Paul Brook wrote:
> > This patch enables USB UHCI global suspend/resume feature. The OS will
> > stop the HC once all ports are suspended. If there is activity on the
> > port(s), an interrupt signalling remote wakeup will be triggered.
> 
> I'm pretty sure this is wrong.  Suspend/resume works based on physical 
> topology, i.e. the resume notification should go to the the port/hub to which 
> the device is connected, not directly to the host controller.

If the host controller is in global suspend state, and resume is
received on any of its root hub ports (given that remote wakeup is
enabled for the given port), the system will be interrupted if interrupt
enable bit is set.

2.1.2 USB STATUS REGISTER
I/O Address: Base + (02-03h)
Default Value: 0000h
Attribute: Read/Write Clear
size: 16 bits
This register indicates pending interrupts and various states of the
Host Controller

Resume Detect. The Host Controller sets this bit to 1 when it receives
a “RESUME” signal from a USB device. This is only valid if the Host
Controller is in a global suspend state (bit 3 of Command register = 1).

2.1.7.1 Behavior Under Global or Selective Suspend Scenarios

Resume will be recognized in the USBSTS register (bit 2) if resume is
received on a suspended or enabled port when the Host Controller is in
the global suspend state (USBCMD register bit 3 is set).

4.2.1 RESUME RECEIVED
This event indicates that the HC received a RESUME signal from a device
on the USB during a global suspend. If
this interrupt is enabled in the HC Interrupt Enable register, a
hardware interrupt will be signaled to the system
allowing the USB to be brought out of the suspend state and returned to
normal operation.

You are correct in that USB HUB emulation does not propagate resume, but
this does not make this patch incorrect.