linux-kernel - Re: [TESTPATCH] xhci: fix usb2 resume timing and races.

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Date:	Wed, 02 Dec 2015 10:50:32 +0200
From:	Mathias Nyman <mathias.nyman@...ux.intel.com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	daniel@...ra.org, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [TESTPATCH] xhci: fix usb2 resume timing and races.

On 01.12.2015 17:47, Alan Stern wrote:
> On Mon, 30 Nov 2015, Mathias Nyman wrote:
>
>> usb2 ports need to signal resume for 20ms before moving to U0 state.
>> Both device and host can initiate resume.
>>
>> On host initated resume port is set to resume state, sleep 20ms,
>> and finally set port to U0 state.
>
> That's an odd approach.  The assumption in usbcore is that the HCD will
> not sleep here.
>
>> On device initated resume a port status interrupt with a port in resume
>> state in issued. The interrupt handler tags a resume_done[port]
>> timestamp with current time + 20ms, and kick roothub timer.
>> Root hub timer requests for port status, finds the port in resume state,
>> checks if resume_done[port] timestamp passed, and set port to U0 state.
>
> ehci-hcd does the same thing, except that it also uses this resume_done
> timestamp with host-initiated resumes.
>
>> There are a few issues with this approach,
>> 1. A host initated resume will also generate a resume event, the event
>>     handler will find the port in resume state, believe it's a device
>>     initated and act accordingly.
>>
>> 2. A port status request might cut the 20ms resume signalling short if a
>>     get_port_status request is handled during the 20ms host resume.
>>     The port will be found in resume state. The timestamp is not set leading
>>     to time_after_eq(jiffoes, timestamp) returning true, as timestamp = 0.
>>     get_port_status will proceed with moving the port to U0.
>>
>> 3. If an error, or anything else happends to the port during device
>>     initated 20ms resume signalling it will leave all device resume
>>     parameters hanging uncleared preventing further resume.
>>
>> Fix this by using the existing resuming_ports bitfield to indicate if
>> resume signalling timing is taken care of.
>> Also check if the resume_done[port] is set  before using it in time
>> comparison. Also clear out any resume signalling related variables if port
>> is not in U0 or Resume state.
>
> Wouldn't it be better to change the host-initiated resume mechanism to
> be consisten with device-initiated resumes?  Or would that be too big a
> change for the time being?
>
> Alan Stern
>

Yes, changing host initiated resume code would make sense.

Hence the comment in the testpatch:

/* Host initated resume doesn't time the resume
  * signalling using resume_done[].
  * It manually sets RESUME state, sleeps 20ms
  * and sets U0 state. This should probably be
  * changed, but not right now, do nothing
  */

I was focusing more on clearing the stale resume related variables and
didn't want to dig into the history of host initiated resume code at that moment.

If ehci-hcd is using the timestamp + kick roothub approach for host resume,
then I don't see why xhci can't do the same.

-Mathias






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