| Sarah Sharp | eab1caf | 2010-04-05 10:55:58 -0700 | [diff] [blame] | 1 | Background |
| 2 | ========== |
| 3 | |
| 4 | Bulk endpoint streams were added in the USB 3.0 specification. Streams allow a |
| 5 | device driver to overload a bulk endpoint so that multiple transfers can be |
| 6 | queued at once. |
| 7 | |
| 8 | Streams are defined in sections 4.4.6.4 and 8.12.1.4 of the Universal Serial Bus |
| 9 | 3.0 specification at http://www.usb.org/developers/docs/ The USB Attached SCSI |
| 10 | Protocol, which uses streams to queue multiple SCSI commands, can be found on |
| 11 | the T10 website (http://t10.org/). |
| 12 | |
| 13 | |
| 14 | Device-side implications |
| 15 | ======================== |
| 16 | |
| 17 | Once a buffer has been queued to a stream ring, the device is notified (through |
| 18 | an out-of-band mechanism on another endpoint) that data is ready for that stream |
| 19 | ID. The device then tells the host which "stream" it wants to start. The host |
| 20 | can also initiate a transfer on a stream without the device asking, but the |
| 21 | device can refuse that transfer. Devices can switch between streams at any |
| 22 | time. |
| 23 | |
| 24 | |
| 25 | Driver implications |
| 26 | =================== |
| 27 | |
| 28 | int usb_alloc_streams(struct usb_interface *interface, |
| 29 | struct usb_host_endpoint **eps, unsigned int num_eps, |
| 30 | unsigned int num_streams, gfp_t mem_flags); |
| 31 | |
| 32 | Device drivers will call this API to request that the host controller driver |
| 33 | allocate memory so the driver can use up to num_streams stream IDs. They must |
| 34 | pass an array of usb_host_endpoints that need to be setup with similar stream |
| 35 | IDs. This is to ensure that a UASP driver will be able to use the same stream |
| 36 | ID for the bulk IN and OUT endpoints used in a Bi-directional command sequence. |
| 37 | |
| 38 | The return value is an error condition (if one of the endpoints doesn't support |
| 39 | streams, or the xHCI driver ran out of memory), or the number of streams the |
| 40 | host controller allocated for this endpoint. The xHCI host controller hardware |
| 41 | declares how many stream IDs it can support, and each bulk endpoint on a |
| 42 | SuperSpeed device will say how many stream IDs it can handle. Therefore, |
| 43 | drivers should be able to deal with being allocated less stream IDs than they |
| 44 | requested. |
| 45 | |
| 46 | Do NOT call this function if you have URBs enqueued for any of the endpoints |
| 47 | passed in as arguments. Do not call this function to request less than two |
| 48 | streams. |
| 49 | |
| 50 | Drivers will only be allowed to call this API once for the same endpoint |
| 51 | without calling usb_free_streams(). This is a simplification for the xHCI host |
| 52 | controller driver, and may change in the future. |
| 53 | |
| 54 | |
| 55 | Picking new Stream IDs to use |
| 56 | ============================ |
| 57 | |
| 58 | Stream ID 0 is reserved, and should not be used to communicate with devices. If |
| 59 | usb_alloc_streams() returns with a value of N, you may use streams 1 though N. |
| 60 | To queue an URB for a specific stream, set the urb->stream_id value. If the |
| 61 | endpoint does not support streams, an error will be returned. |
| 62 | |
| 63 | Note that new API to choose the next stream ID will have to be added if the xHCI |
| 64 | driver supports secondary stream IDs. |
| 65 | |
| 66 | |
| 67 | Clean up |
| 68 | ======== |
| 69 | |
| 70 | If a driver wishes to stop using streams to communicate with the device, it |
| 71 | should call |
| 72 | |
| 73 | void usb_free_streams(struct usb_interface *interface, |
| 74 | struct usb_host_endpoint **eps, unsigned int num_eps, |
| 75 | gfp_t mem_flags); |
| 76 | |
| 77 | All stream IDs will be deallocated when the driver releases the interface, to |
| 78 | ensure that drivers that don't support streams will be able to use the endpoint. |