You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
83 lines
3.2 KiB
83 lines
3.2 KiB
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.
|
|
|