84 lines
3.2 KiB
ReStructuredText
84 lines
3.2 KiB
ReStructuredText
USB bulk streams
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~~~~~~~~~~~~~~~~
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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 https://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 (https://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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::
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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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