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authorMauro Carvalho Chehab <mchehab@s-opensource.com>2017-04-05 16:23:03 +0300
committerJonathan Corbet <corbet@lwn.net>2017-04-11 23:38:56 +0300
commit401c7be2bad9eea68c1a660d54d1caa00b3098e4 (patch)
treef74c39efc2bf7e91483f0bab5d2cad0169c2d443 /Documentation/driver-api/usb
parent79e0c2e6d4a382a7ac80cf082e3ca60bd42ab475 (diff)
downloadlinux-401c7be2bad9eea68c1a660d54d1caa00b3098e4.tar.xz
usb/bulk-streams.txt: convert to ReST and add to driver-api book
This document describe some USB core functions. Add it to the driver-api book. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation/driver-api/usb')
-rw-r--r--Documentation/driver-api/usb/bulk-streams.rst83
-rw-r--r--Documentation/driver-api/usb/index.rst1
2 files changed, 84 insertions, 0 deletions
diff --git a/Documentation/driver-api/usb/bulk-streams.rst b/Documentation/driver-api/usb/bulk-streams.rst
new file mode 100644
index 000000000000..99b515babdeb
--- /dev/null
+++ b/Documentation/driver-api/usb/bulk-streams.rst
@@ -0,0 +1,83 @@
+USB bulk streams
+~~~~~~~~~~~~~~~~
+
+Background
+==========
+
+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 4.4.6.4 and 8.12.1.4 of the Universal Serial Bus
+3.0 specification at http://www.usb.org/developers/docs/ The USB Attached SCSI
+Protocol, which uses streams to queue multiple SCSI commands, can be found on
+the T10 website (http://t10.org/).
+
+
+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
+time.
+
+
+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
+requested.
+
+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
+streams.
+
+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.
diff --git a/Documentation/driver-api/usb/index.rst b/Documentation/driver-api/usb/index.rst
index 5dfb04b2d730..6fe7611f7332 100644
--- a/Documentation/driver-api/usb/index.rst
+++ b/Documentation/driver-api/usb/index.rst
@@ -7,6 +7,7 @@ Linux USB API
usb
gadget
anchors
+ bulk-streams
writing_usb_driver
writing_musb_glue_layer