linux-kernel - Re: [TESTPATCH v2] xhci: fix usb2 resume timing and races.

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Date:	Tue, 08 Dec 2015 13:16:30 +0200
From:	Mathias Nyman <mathias.nyman@...ux.intel.com>
To:	Daniel J Blueman <daniel@...ra.org>
CC:	USB list <linux-usb@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [TESTPATCH v2] xhci: fix usb2 resume timing and races.

On 05.12.2015 06:02, Daniel J Blueman wrote:
> On 1 December 2015 at 16:26, Mathias Nyman
> <mathias.nyman@...ux.intel.com> 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.
>>
>> 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.
>>
>> 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.
>>
>> v2. fix parentheses when checking for uncleared resume variables.
>> we want: if ((unclear1 OR unclear2 ) AND !in_resume AND !in_U3) { .. }
>>
>> Signed-off-by: Mathias Nyman <mathias.nyman@...ux.intel.com>
>
> Excellent; this correctly prevents the cyclic chain of suspend
> attempts, resolving the issue.
>
> Tested-by: Daniel J Blueman <daniel@...ra.org>
>
> Thanks Mathias!
>    Daniel
>

Thanks for testing it
I'll do some minor cleanups and add it to the queue

-Mathias
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