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Bulk endpoint streams were added in the USB 3.0 specification. Streams
allow a device driver to overload a bulk endpoint so that multiple
transfers can be queued at once.
The device then decides which transfer it wants to work on first, and can
queue part of a transfer before it switches to a new stream. All this
switching is invisible to the device driver, which just gets a completion
for the URB. Drivers that use streams must be able to handle URBs
completing in a different order than they were submitted to the endpoint.
This requires adding new API to set up xHCI data structures to support
multiple queues ("stream rings") per endpoint. Drivers will allocate a
number of stream IDs before enqueueing URBs to the bulk endpoints of the
device, and free the stream IDs in their disconnect function. See
Documentation/usb/bulk-streams.txt for details.
The new mass storage device class, USB Attached SCSI Protocol (UASP), uses
these streams API.
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
79 lines
3.2 KiB
Plaintext
79 lines
3.2 KiB
Plaintext
Background
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==========
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Bulk endpoint streams were added in the USB 3.0 specification. Streams allow a
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device driver to overload a bulk endpoint so that multiple transfers can be
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queued at once.
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Streams are defined in sections 4.4.6.4 and 8.12.1.4 of the Universal Serial Bus
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3.0 specification at http://www.usb.org/developers/docs/ The USB Attached SCSI
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Protocol, which uses streams to queue multiple SCSI commands, can be found on
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the T10 website (http://t10.org/).
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Device-side implications
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========================
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Once a buffer has been queued to a stream ring, the device is notified (through
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an out-of-band mechanism on another endpoint) that data is ready for that stream
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ID. The device then tells the host which "stream" it wants to start. The host
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can also initiate a transfer on a stream without the device asking, but the
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device can refuse that transfer. Devices can switch between streams at any
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time.
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Driver implications
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===================
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int usb_alloc_streams(struct usb_interface *interface,
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struct usb_host_endpoint **eps, unsigned int num_eps,
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unsigned int num_streams, gfp_t mem_flags);
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Device drivers will call this API to request that the host controller driver
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allocate memory so the driver can use up to num_streams stream IDs. They must
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pass an array of usb_host_endpoints that need to be setup with similar stream
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IDs. This is to ensure that a UASP driver will be able to use the same stream
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ID for the bulk IN and OUT endpoints used in a Bi-directional command sequence.
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The return value is an error condition (if one of the endpoints doesn't support
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streams, or the xHCI driver ran out of memory), or the number of streams the
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host controller allocated for this endpoint. The xHCI host controller hardware
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declares how many stream IDs it can support, and each bulk endpoint on a
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SuperSpeed device will say how many stream IDs it can handle. Therefore,
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drivers should be able to deal with being allocated less stream IDs than they
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requested.
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Do NOT call this function if you have URBs enqueued for any of the endpoints
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passed in as arguments. Do not call this function to request less than two
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streams.
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Drivers will only be allowed to call this API once for the same endpoint
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without calling usb_free_streams(). This is a simplification for the xHCI host
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controller driver, and may change in the future.
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Picking new Stream IDs to use
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============================
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Stream ID 0 is reserved, and should not be used to communicate with devices. If
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usb_alloc_streams() returns with a value of N, you may use streams 1 though N.
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To queue an URB for a specific stream, set the urb->stream_id value. If the
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endpoint does not support streams, an error will be returned.
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Note that new API to choose the next stream ID will have to be added if the xHCI
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driver supports secondary stream IDs.
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Clean up
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========
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If a driver wishes to stop using streams to communicate with the device, it
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should call
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void usb_free_streams(struct usb_interface *interface,
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struct usb_host_endpoint **eps, unsigned int num_eps,
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gfp_t mem_flags);
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All stream IDs will be deallocated when the driver releases the interface, to
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ensure that drivers that don't support streams will be able to use the endpoint.
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