USB 3.0 devices may optionally support a new feature called Latency
Tolerance Messaging. If both the xHCI host controller and the device
support LTM, it should be turned on in order to give the system hardware
a better clue about the latency tolerance values of its PCI devices.
Once a Set Feature request to enable LTM is received, the USB 3.0 device
will begin to send LTM updates as its buffers fill or empty, and it can
tolerate more or less latency.
The USB 3.0 spec, section C.4.2 says that LTM should be disabled just
before the device is placed into suspend. Then the device will send an
updated LTM notification, so that the system doesn't think it should
remain in an active state in order to satisfy the latency requirements
of the suspended device.
The Set and Clear Feature LTM enable command can only be sent to a
configured device. The device will respond with an error if that
command is sent while it is in the Default or Addressed state. Make
sure to check udev->actconfig in usb_enable_ltm() and usb_disable_ltm(),
and don't send those commands when the device is unconfigured.
LTM should be enabled once a new configuration is installed in
usb_set_configuration(). If we end up sending duplicate Set Feature LTM
Enable commands on a switch from one installed configuration to another
configuration, that should be harmless.
Make sure that LTM is disabled before the device is unconfigured in
usb_disable_device(). If no drivers are bound to the device, it doesn't
make sense to allow the device to control the latency tolerance of the
xHCI host controller.
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
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