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    SubjectRe: [TESTPATCH v2] xhci: fix usb2 resume timing and races.
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    On 1 December 2015 at 16:26, Mathias Nyman
    <mathias.nyman@linux.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@linux.intel.com>

    Excellent; this correctly prevents the cyclic chain of suspend
    attempts, resolving the issue.

    Tested-by: Daniel J Blueman <daniel@quora.org>

    Thanks Mathias!
    Daniel


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