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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.
-Streams are defined in sections and of the Universal Serial Bus
-3.0 specification at The USB Attached SCSI
-Protocol, which uses streams to queue multiple SCSI commands, can be found on
-the T10 website (
-Device-side implications
-Once a buffer has been queued to a stream ring, the device is notified (through
-an out-of-band mechanism on another endpoint) that data is ready for that stream
-ID. The device then tells the host which "stream" it wants to start. The host
-can also initiate a transfer on a stream without the device asking, but the
-device can refuse that transfer. Devices can switch between streams at any
-Driver implications
-int usb_alloc_streams(struct usb_interface *interface,
- struct usb_host_endpoint **eps, unsigned int num_eps,
- unsigned int num_streams, gfp_t mem_flags);
-Device drivers will call this API to request that the host controller driver
-allocate memory so the driver can use up to num_streams stream IDs. They must
-pass an array of usb_host_endpoints that need to be setup with similar stream
-IDs. This is to ensure that a UASP driver will be able to use the same stream
-ID for the bulk IN and OUT endpoints used in a Bi-directional command sequence.
-The return value is an error condition (if one of the endpoints doesn't support
-streams, or the xHCI driver ran out of memory), or the number of streams the
-host controller allocated for this endpoint. The xHCI host controller hardware
-declares how many stream IDs it can support, and each bulk endpoint on a
-SuperSpeed device will say how many stream IDs it can handle. Therefore,
-drivers should be able to deal with being allocated less stream IDs than they
-Do NOT call this function if you have URBs enqueued for any of the endpoints
-passed in as arguments. Do not call this function to request less than two
-Drivers will only be allowed to call this API once for the same endpoint
-without calling usb_free_streams(). This is a simplification for the xHCI host
-controller driver, and may change in the future.
-Picking new Stream IDs to use
-Stream ID 0 is reserved, and should not be used to communicate with devices. If
-usb_alloc_streams() returns with a value of N, you may use streams 1 though N.
-To queue an URB for a specific stream, set the urb->stream_id value. If the
-endpoint does not support streams, an error will be returned.
-Note that new API to choose the next stream ID will have to be added if the xHCI
-driver supports secondary stream IDs.
-Clean up
-If a driver wishes to stop using streams to communicate with the device, it
-should call
-void usb_free_streams(struct usb_interface *interface,
- struct usb_host_endpoint **eps, unsigned int num_eps,
- gfp_t mem_flags);
-All stream IDs will be deallocated when the driver releases the interface, to
-ensure that drivers that don't support streams will be able to use the endpoint.