Re: usb:xhci: support disable usb2 LPM Remote Wakeup

[Mathias Nyman <mathias.nyman@linux.intel.com>]

On 12.12.2016 06:00, Thang Q. Nguyen wrote:
> On Sat, Dec 10, 2016 at 4:36 AM, Rob Herring <robh@kernel.org> wrote:
>> On Sun, Dec 04, 2016 at 07:42:01PM +0700, Thang Q. Nguyen wrote:
>>> From: Thang Nguyen <tqnguyen@apm.com>
>>>
>>> As per USB 2.0 link power management addendum ECN, table 1-2, page 4,
>>> device or host initiated via resume signaling; device-initiated resumes
>>> can be optionally enabled/disabled by software. This patch adds support
>>> to control enabling the USB2 RWE feature via DT/ACPI attribute.
>>>
>>> Signed-off-by: Vu Nguyen <vnguyen@apm.com>
>>> Signed-off-by: Thang Nguyen <tqnguyen@apm.com>
>>> ---
>>>   Documentation/devicetree/bindings/usb/usb-xhci.txt | 1 +
>>>   drivers/usb/host/xhci-plat.c                       | 3 +++
>>>   drivers/usb/host/xhci.c                            | 5 ++++-
>>>   drivers/usb/host/xhci.h                            | 1 +
>>>   4 files changed, 9 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/Documentation/devicetree/bindings/usb/usb-xhci.txt b/Documentation/devicetree/bindings/usb/usb-xhci.txt
>>> index 966885c..9b4cd14 100644
>>> --- a/Documentation/devicetree/bindings/usb/usb-xhci.txt
>>> +++ b/Documentation/devicetree/bindings/usb/usb-xhci.txt
>>> @@ -25,6 +25,7 @@ Required properties:
>>>
>>>   Optional properties:
>>>     - clocks: reference to a clock
>>> +  - usb2-rwe-disable: disable USB2 LPM Remote Wakeup capable
>>
>> Remote wakeup has been around since USB 1.0 days. Does this need to be
>> USB2 or XHCI specific?
> This is XHCI specific. Per XHCI specification 1.1, remote wakeup is
> optional for XHCI 1.0 and required for XHCI 1.1. This patch provides
> ability for software to disable RWE for USB2 in XHCI1.0 controller.

I think I understand what's going on.

USB:
   
The good old USB2 suspend is called L2. Device enters it after 3ms if there is no link activity.
If a device can remote wakeup (RWE) it's stated in the descriptor. RWE can be turned on
of off using standard SET/CLEAR Fature requests

The LPM L1 USB2 state again is entered with a LPM extended transaction to avoid the
3ms wait before powersaving. L1 state is exit can be done with a simialr RWE as L2 resume.
The RWE from L1 can turned on/off using a bit in the LPM extended transaction.

XHCI:

Specs say that if the device supports RWE we should enable it for LPM L1 exit as well.
This is done by setting the RWE (LPM L1) bit in PORTPMSC register. This bit only affect LPM L1 remote
wake. see 4.23.5.1.1.1

The issue might be that xhci driver never check if the device actually supports RWE, we always
set the PORTPMSC RWE  (for LPM L1) bit.

How about checking something like udev->do_remote_wakeup and setting and setting the bit
based on that.

The function that you are changing,  xhci_set_usb2_hardware_lpm() should only be used if
host has Hardware LPM Cabaility bit (HLC) set for that USB2 port in the
USB 2.0 xHCI Supported Protocol Capability.
Host that don't supprt LPM won't have that set. See xhci 7.2.2.1.3.2
  
-Mathias