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Subject: Re: [PATCH 1/2] xhci: Suspend ports to U3 directly from U1 or U2
To:     Kai-Heng Feng <kai.heng.feng@canonical.com>,
        Alan Stern <stern@rowland.harvard.edu>
Cc:     Mathias Nyman <mathias.nyman@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "open list:USB XHCI DRIVER" <linux-usb@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
References: <20200610064231.9454-1-kai.heng.feng@canonical.com>
 <20200610143220.GC11727@rowland.harvard.edu>
 <591D2A1F-9645-4B0B-896C-99544F06DFAA@canonical.com>
From:   Mathias Nyman <mathias.nyman@linux.intel.com>
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Message-ID: <9d7842de-9813-becd-80a0-a422e59c1e94@linux.intel.com>
Date:   Wed, 10 Jun 2020 18:58:41 +0300
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On 10.6.2020 18.43, Kai-Heng Feng wrote:
> 
> 
>> On Jun 10, 2020, at 22:32, Alan Stern <stern@rowland.harvard.edu> wrote:
>>
>> On Wed, Jun 10, 2020 at 02:42:30PM +0800, Kai-Heng Feng wrote:
>>> xHCI spec "4.15.1 Port Suspend" states that port can be put to U3 as long
>>> as Enabled bit is set and from U0, U1 or U2 state.
>>>
>>> Currently only USB_PORT_FEAT_LINK_STATE puts port to U3 directly, let's
>>> do the same for USB_PORT_FEAT_SUSPEND and bus suspend case.
>>>
>>> This is particularly useful for USB2 devices, which may take a very long
>>> time to switch USB2 LPM on and off.
>>
>> Have these two patches been tested with a variety of USB-2.0 and USB-2.1 
>> devices?
> 
> I tested some laptops around and they work fine.
> Only internally connected USB devices like USB Bluetooth and USB Camera have USB2 LPM enabled, so this patch won't affect external connected devices.
> 

Took a fresh look at the USB2 side and it's not as clear as the USB3 case, where
we know the hub must support transition to U3 from any other state. [1]

Supporting link state transition to U3 (USB2 L2) from any other U state for USB2 seems
to be xHCI specific feature. xHC hardware will make sure it goes via the U0 state.

I have no clue about other hosts (or hubs), USB2 LPM ECN just shows that link
state transitions to L1 or L2 should always goes via L0.
It's possible this has to be done in software by disabling USB2 LPM before suspending the device.

So I guess the original suggestion to wait for link state to reach U0 before
is a better solution. Sorry about my hasty suggestion.

Kai-Heng, does it help if you fist manually set the link to U0 before disabling
USB2 LPM (set PLS to 0 before clearing the HLE bit). Does it transition to U0
any faster, or get rid of the extra port event with PLC:U0?

-Mathias

 
[1]
    USB3.1 spec (10.16.2.10) Set Port feature:   
   "If the value is 3, then host software wants to selectively suspend the
    device connected to this port. The hub shall transition the link to U3
    from any of the other U states using allowed link state transitions.
    If the port is not already in the U0 state, then it shall transition the
    port to the U0 state and then initiate the transition to U3.