# The Linux-USB Host Side API

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## Introduction to USB on Linux

A Universal Serial Bus (USB) is used to connect a host, such as a PC or workstation, to a number of peripheral devices. USB uses a tree structure, with the host as the root (the system's master), hubs as interior nodes, and peripherals as leaves (and slaves). Modern PCs support several such trees of USB devices, usually one USB 2.0 tree (480 Mbit/sec each) with a few USB 1.1 trees (12 Mbit/sec each) that are used when you connect a USB 1.1 device directly to the machine's "root hub".

That master/slave asymmetry was designed-in for a number of reasons, one being ease of use. It is not physically possible to assemble (legal) USB cables incorrectly: all upstream "to the host" connectors are the rectangular type (matching the sockets on root hubs), and all downstream connectors are the squarish type (or they are built into the peripheral). Also, the host software doesn't need to deal with distributed auto-configuration since the pre-designated master node manages all that. And finally, at the electrical level, bus protocol overhead is reduced by eliminating arbitration and moving scheduling into the host software.

USB 1.0 was announced in January 1996 and was revised as USB 1.1 (with improvements in hub specification and support for interrupt-out transfers) in September 1998. USB 2.0 was released in April 2000, adding high-speed transfers and transaction-translating hubs (used for USB 1.1 and 1.0 backward compatibility).

Kernel developers added USB support to Linux early in the 2.2 kernel series, shortly before 2.3 development forked. Updates from 2.3 were regularly folded back into 2.2 releases, which improved reliability and brought `/sbin/hotplug` support as well more drivers. Such improvements were continued in the 2.5 kernel series, where they added USB 2.0 support, improved performance, and made the host controller drivers (HCDs) more consistent. They also simplified the API (to make bugs less likely) and added internal "kerneldoc" documentation.

Linux can run inside USB devices as well as on the hosts that control the devices. But USB device drivers running inside those peripherals don't do the same things as the ones running inside hosts, so they've been given a different name: gadget drivers. This document does not cover gadget drivers.

## USB Host-Side API Model

Host-side drivers for USB devices talk to the "usbcore" APIs. There are two. One is intended for general-purpose drivers (exposed through driver frameworks), and the other is for drivers that are part of the core. Such core drivers include the hub driver (which manages trees of USB devices) and several different kinds of host controller drivers, which control individual busses.

The device model seen by USB drivers is relatively complex.

- USB supports four kinds of data transfers (control, bulk, interrupt, and isochronous). Two of them (control and bulk) use bandwidth as it's available, while the other two (interrupt and isochronous) are scheduled to provide guaranteed bandwidth.

- The device description model includes one or more "configurations" per device, only one of which is active at a time. Devices that are capable of high-speed operation must also support full-speed configurations, along with a way to ask about the "other speed" configurations which might be used.

- Configurations have one or more "interfaces", each of which may have "alternate settings". Interfaces may be standardized by USB "Class" specifications, or may be specific to a vendor or device.

  USB device drivers actually bind to interfaces, not devices. Think of them as "interface drivers", though you may not see many devices where the distinction is important. Most USB devices are simple, with only one configuration, one interface, and one alternate setting.

- Interfaces have one or more "endpoints", each of which supports one type and direction of data transfer such as "bulk out" or "interrupt in". The entire configuration may have up to sixteen endpoints in each direction, allocated as needed among all the interfaces.

- Data transfer on USB is packetized; each endpoint has a maximum packet size. Drivers must often be aware of conventions such as flagging the end of bulk transfers using "short" (including zero length) packets.

- The Linux USB API supports synchronous calls for control and bulk messages. It also supports asynchnous calls for all kinds of data transfer, using request structures called "URBs" (USB Request Blocks).

Accordingly, the USB Core API exposed to device drivers covers quite a lot of territory. You'll probably need to consult the USB 2.0 specification, available online from www.usb.org at no cost, as well as class or device specifications.

The only host-side drivers that actually touch hardware (reading/writing registers, handling IRQs, and so on) are the HCDs. In theory, all HCDs provide the same functionality through the same API. In practice, that's becoming more true on the 2.5 kernels, but there are still differences that crop up especially with fault handling. Different controllers don't necessarily report the same aspects of failures, and recovery from faults (including software-induced ones like unlinking an URB) isn't yet fully consistent. Device driver authors should make a point of doing disconnect testing (while the device is active) with each different host controller driver, to make sure drivers don't have bugs of their own as well as to make sure they aren't relying on some HCD-specific behavior. (You will need external USB 1.1 and/or USB 2.0 hubs to perform all those tests.)

## USB-Standard Types

In `<linux/usb/ch9.h>` you will find the USB data types defined in chapter 9 of the USB specification. These data types are used throughout USB, and in APIs including this host side API, gadget APIs, and usbfs.

!Iinclude/linux/usb/ch9.h

## Host-Side Data Types and Macros

The host side API exposes several layers to drivers, some of which are more necessary than others. These support lifecycle models for host side drivers and devices, and support passing buffers through usbcore to some HCD that performs the I/O for the device driver.

!Iinclude/linux/usb.h

## USB Core APIs

There are two basic I/O models in the USB API. The most elemental one is asynchronous: drivers submit requests in the form of an URB, and the URB's completion callback handle the next step. All USB transfer types support that model, although there are special cases for control URBs (which always have setup and status stages, but may not have a data stage) and isochronous URBs (which allow large packets and include per-packet fault reports). Built on top of that is synchronous API support, where a driver calls a routine that allocates one or more URBs, submits them, and waits until they complete. There are synchronous wrappers for single-buffer control and bulk transfers (which are awkward to use in some driver disconnect scenarios), and for scatterlist based streaming i/o (bulk or interrupt).

USB drivers need to provide buffers that can be used for DMA, although they don't necessarily need to provide the DMA mapping themselves. There are APIs to use used when allocating DMA buffers, which can prevent use of bounce buffers on some systems. In some cases, drivers may be able to rely on 64bit DMA to eliminate another kind of bounce buffer.

!Edrivers/usb/core/urb.c !Edrivers/usb/core/message.c !Edrivers/usb/core/file.c !Edrivers/usb/core/driver.c !Edrivers/usb/core/usb.c !Edrivers/usb/core/hub.c

## Host Controller APIs

These APIs are only for use by host controller drivers, most of which implement standard register interfaces such as EHCI, OHCI, or UHCI. UHCI was one of the first interfaces, designed by Intel and also used by VIA; it doesn't do much in hardware. OHCI was designed later, to have the hardware do more work (bigger transfers, tracking protocol state, and so on). EHCI was designed with USB 2.0; its design has features that resemble OHCI (hardware does much more work) as well as UHCI (some parts of ISO support, TD list processing).

There are host controllers other than the "big three", although most PCI based controllers (and a few non-PCI based ones) use one of those interfaces. Not all host controllers use DMA; some use PIO, and there is also a simulator.

The same basic APIs are available to drivers for all those controllers. For historical reasons they are in two layers: struct usb_bus is a rather thin layer that became available in the 2.2 kernels, while struct usb_hcd is a more featureful layer (available in later 2.4 kernels and in 2.5) that lets HCDs share common code, to shrink driver size and significantly reduce hcd-specific behaviors.

!Edrivers/usb/core/hcd.c !Edrivers/usb/core/hcd-pci.c !Idrivers/usb/core/buffer.c

## The USB Filesystem (usbfs)

This chapter presents the Linux usbfs. You may prefer to avoid writing new kernel code for your USB driver; that's the problem that usbfs set out to solve. User mode device drivers are usually packaged as applications or libraries, and may use usbfs through some programming library that wraps it. Such libraries include libusb for C/C++, and jUSB for Java.

### Unfinished

This particular documentation is incomplete, especially with respect to the asynchronous mode. As of kernel 2.5.66 the code and this (new) documentation need to be cross-reviewed.

Configure usbfs into Linux kernels by enabling the USB filesystem option (CONFIG_USB_DEVICEFS), and you get basic support for user mode USB device drivers. Until relatively recently it was often (confusingly) called usbdevfs although it wasn't solving what devfs was. Every USB device will appear in usbfs, regardless of whether or not it has a kernel driver.

### What files are in "usbfs"?

Conventionally mounted at `/proc/bus/usb`, usbfs features include:

- `/proc/bus/usb/devices` ... a text file showing each of the USB devices on known to the kernel, and their configuration descriptors. You can also poll() this to learn about new devices.

- `/proc/bus/usb/BBB/DDD` ... magic files exposing the each device's configuration descriptors, and supporting a series of ioctls for making device requests, including I/O to devices. (Purely for access by programs.)

Each bus is given a number (BBB) based on when it was enumerated; within each bus, each device is given a similar number (DDD). Those BBB/DDD paths are not "stable" identifiers; expect them to change even if you always leave the devices plugged in to the same hub port. Don't even think of saving these in application configuration files. Stable identifiers are available, for user mode applications that want to use them. HID and networking devices expose these stable IDs, so that for example you can be sure that you told the right UPS to power down its second server. "usbfs" doesn't (yet) expose those IDs.

### Mounting and Access Control

There are a number of mount options for usbfs, which will be of most interest to you if you need to override the default access control policy. That policy is that only root may read or write device files (`/proc/bus/BBB/DDD`) although anyone may read the `devices` or `drivers` files. I/O requests to the device also need the CAP_SYS_RAWIO capability,

The significance of that is that by default, all user mode device drivers need super-user privileges. You can change modes or ownership in a driver setup when the device hotplugs, or maye just start the driver right then, as a privileged server (or some activity within one). That's the most secure approach for multi-user systems, but for single user systems ("trusted" by that user) it's more convenient just to grant everyone all access (using the devmode=0666 option) so the driver can start whenever it's needed.

The mount options for usbfs, usable in /etc/fstab or in command line invocations of mount, are:

busgid=NNNNN

  Controls the GID used for the /proc/bus/usb/BBB directories. (Default: 0)

busmode=MMM

  Controls the file mode used for the /proc/bus/usb/BBB directories. (Default: 0555)

busuid=NNNNN

  Controls the UID used for the /proc/bus/usb/BBB directories. (Default: 0)

devgid=NNNNN

  Controls the GID used for the /proc/bus/usb/BBB/DDD files. (Default: 0)

devmode=MMM

  Controls the file mode used for the /proc/bus/usb/BBB/DDD files. (Default: 0644)

devuid=NNNNN

  Controls the UID used for the /proc/bus/usb/BBB/DDD files. (Default: 0)

listgid=NNNNN

  Controls the GID used for the /proc/bus/usb/devices and drivers files. (Default: 0)

listmode=MMM

  Controls the file mode used for the /proc/bus/usb/devices and drivers files. (Default: 0444)

listuid=NNNNN

  Controls the UID used for the /proc/bus/usb/devices and drivers files. (Default: 0)

Note that many Linux distributions hard-wire the mount options for usbfs in their init scripts, such as `/etc/rc.d/rc.sysinit`, rather than making it easy to set this per-system policy in `/etc/fstab`.

### /proc/bus/usb/devices

This file is handy for status viewing tools in user mode, which can scan the text format and ignore most of it. More detailed device status (including class and vendor status) is available from device-specific files. For information about the current format of this file, see the `Documentation/usb/proc_usb_info.txt` file in your Linux kernel sources.

This file, in combination with the poll() system call, can also be used to detect when devices are added or removed:

    int fd;
    struct pollfd pfd;

    fd = open("/proc/bus/usb/devices", O_RDONLY);
    pfd = { fd, POLLIN, 0 };
    for (;;) {
    	/* The first time through, this call will return immediately. */
    	poll(&pfd, 1, -1);

    	/* To see what's changed, compare the file's previous and current
    	   contents or scan the filesystem.  (Scanning is more precise.) */
    }

Note that this behavior is intended to be used for informational and debug purposes. It would be more appropriate to use programs such as udev or HAL to initialize a device or start a user-mode helper program, for instance.

### /proc/bus/usb/BBB/DDD

Use these files in one of these basic ways:

They can be read, producing first the device descriptor (18 bytes) and then the descriptors for the current configuration. See the USB 2.0 spec for details about those binary data formats. You'll need to convert most multibyte values from little endian format to your native host byte order, although a few of the fields in the device descriptor (both of the BCD-encoded fields, and the vendor and product IDs) will be byteswapped for you. Note that configuration descriptors include descriptors for interfaces, altsettings, endpoints, and maybe additional class descriptors.

Perform USB operations using ioctl() requests to make endpoint I/O requests (synchronously or asynchronously) or manage the device. These requests need the CAP_SYS_RAWIO capability, as well as filesystem access permissions. Only one ioctl request can be made on one of these device files at a time. This means that if you are synchronously reading an endpoint from one thread, you won't be able to write to a different endpoint from another thread until the read completes. This works for half duplex protocols, but otherwise you'd use asynchronous i/o requests.

### Life Cycle of User Mode Drivers

Such a driver first needs to find a device file for a device it knows how to handle. Maybe it was told about it because a `/sbin/hotplug` event handling agent chose that driver to handle the new device. Or maybe it's an application that scans all the /proc/bus/usb device files, and ignores most devices. In either case, it should `read()` all the descriptors from the device file, and check them against what it knows how to handle. It might just reject everything except a particular vendor and product ID, or need a more complex policy.

Never assume there will only be one such device on the system at a time! If your code can't handle more than one device at a time, at least detect when there's more than one, and have your users choose which device to use.

Once your user mode driver knows what device to use, it interacts with it in either of two styles. The simple style is to make only control requests; some devices don't need more complex interactions than those. (An example might be software using vendor-specific control requests for some initialization or configuration tasks, with a kernel driver for the rest.)

More likely, you need a more complex style driver: one using non-control endpoints, reading or writing data and claiming exclusive use of an interface. Bulk transfers are easiest to use, but only their sibling interrupt transfers work with low speed devices. Both interrupt and isochronous transfers offer service guarantees because their bandwidth is reserved. Such "periodic" transfers are awkward to use through usbfs, unless you're using the asynchronous calls. However, interrupt transfers can also be used in a synchronous "one shot" style.

Your user-mode driver should never need to worry about cleaning up request state when the device is disconnected, although it should close its open file descriptors as soon as it starts seeing the ENODEV errors.

### The ioctl() Requests

To use these ioctls, you need to include the following headers in your userspace program:

    #include <linux/usb.h>
    #include <linux/usbdevice_fs.h>
    #include <asm/byteorder.h>

The standard USB device model requests, from "Chapter 9" of the USB 2.0 specification, are automatically included from the `<linux/usb/ch9.h>` header.

Unless noted otherwise, the ioctl requests described here will update the modification time on the usbfs file to which they are applied (unless they fail). A return of zero indicates success; otherwise, a standard USB error code is returned. (These are documented in `Documentation/usb/error-codes.txt` in your kernel sources.)

Each of these files multiplexes access to several I/O streams, one per endpoint. Each device has one control endpoint (endpoint zero) which supports a limited RPC style RPC access. Devices are configured by khubd (in the kernel) setting a device-wide configuration that affects things like power consumption and basic functionality. The endpoints are part of USB interfaces, which may have altsettings affecting things like which endpoints are available. Many devices only have a single configuration and interface, so drivers for them will ignore configurations and altsettings.

#### Management/Status Requests

A number of usbfs requests don't deal very directly with device I/O. They mostly relate to device management and status. These are all synchronous requests.

USBDEVFS_CLAIMINTERFACE

  This is used to force usbfs to claim a specific interface, which has not previously been claimed by usbfs or any other kernel driver. The ioctl parameter is an integer holding the number of the interface (bInterfaceNumber from descriptor).

  Note that if your driver doesn't claim an interface before trying to use one of its endpoints, and no other driver has bound to it, then the interface is automatically claimed by usbfs.

  This claim will be released by a RELEASEINTERFACE ioctl, or by closing the file descriptor. File modification time is not updated by this request.

USBDEVFS_CONNECTINFO

  Says whether the device is lowspeed. The ioctl parameter points to a structure like this:

      struct usbdevfs_connectinfo {
              unsigned int   devnum;
              unsigned char  slow;
      };

  File modification time is not updated by this request.

  You can't tell whether a "not slow" device is connected at high speed (480 MBit/sec) or just full speed (12 MBit/sec). You should know the devnum value already, it's the DDD value of the device file name.

USBDEVFS_GETDRIVER

  Returns the name of the kernel driver bound to a given interface (a string). Parameter is a pointer to this structure, which is modified:

      struct usbdevfs_getdriver {
              unsigned int  interface;
              char          driver[USBDEVFS_MAXDRIVERNAME + 1];
      };

  File modification time is not updated by this request.
USBDEVFS_IOCTL

  Passes a request from userspace through to a kernel driver that has an ioctl entry in the struct usb_driver it registered.

      struct usbdevfs_ioctl {
              int     ifno;
              int     ioctl_code;
              void    *data;
      };

      /* user mode call looks like this.
       * 'request' becomes the driver->ioctl() 'code' parameter.
       * the size of 'param' is encoded in 'request', and that data
       * is copied to or from the driver->ioctl() 'buf' parameter.
       */
      static int
      usbdev_ioctl (int fd, int ifno, unsigned request, void *param)
      {
              struct usbdevfs_ioctl	wrapper;

              wrapper.ifno = ifno;
              wrapper.ioctl_code = request;
              wrapper.data = param;

              return ioctl (fd, USBDEVFS_IOCTL, &wrapper);
      }

  File modification time is not updated by this request.

  This request lets kernel drivers talk to user mode code through filesystem operations even when they don't create a character or block special device. It's also been used to do things like ask devices what device special file should be used. Two pre-defined ioctls are used to disconnect and reconnect kernel drivers, so that user mode code can completely manage binding and configuration of devices.

USBDEVFS_RELEASEINTERFACE

  This is used to release the claim usbfs made on interface, either implicitly or because of a USBDEVFS_CLAIMINTERFACE call, before the file descriptor is closed. The ioctl parameter is an integer holding the number of the interface (bInterfaceNumber from descriptor); File modification time is not updated by this request.

  No security check is made to ensure that the task which made the claim is the one which is releasing it. This means that user mode driver may interfere other ones.

USBDEVFS_RESETEP

  Resets the data toggle value for an endpoint (bulk or interrupt) to DATA0. The ioctl parameter is an integer endpoint number (1 to 15, as identified in the endpoint descriptor), with USB_DIR_IN added if the device's endpoint sends data to the host.

  Avoid using this request. It should probably be removed. Using it typically means the device and driver will lose toggle synchronization. If you really lost synchronization, you likely need to completely handshake with the device, using a request like CLEAR_HALT or SET_INTERFACE.

#### Synchronous I/O Support

Synchronous requests involve the kernel blocking until the user mode request completes, either by finishing successfully or by reporting an error. In most cases this is the simplest way to use usbfs, although as noted above it does prevent performing I/O to more than one endpoint at a time.

USBDEVFS_BULK

  Issues a bulk read or write request to the device. The ioctl parameter is a pointer to this structure:

      struct usbdevfs_bulktransfer {
              unsigned int  ep;
              unsigned int  len;
              unsigned int  timeout; /* in milliseconds */
              void          *data;
      };

  The "ep" value identifies a bulk endpoint number (1 to 15, as identified in an endpoint descriptor), masked with USB_DIR_IN when referring to an endpoint which sends data to the host from the device. The length of the data buffer is identified by "len"; Recent kernels support requests up to about 128KBytes. FIXME say how read length is returned, and how short reads are handled..

USBDEVFS_CLEAR_HALT

  Clears endpoint halt (stall) and resets the endpoint toggle. This is only meaningful for bulk or interrupt endpoints. The ioctl parameter is an integer endpoint number (1 to 15, as identified in an endpoint descriptor), masked with USB_DIR_IN when referring to an endpoint which sends data to the host from the device.

  Use this on bulk or interrupt endpoints which have stalled, returning -EPIPE status to a data transfer request. Do not issue the control request directly, since that could invalidate the host's record of the data toggle.

USBDEVFS_CONTROL

  Issues a control request to the device. The ioctl parameter points to a structure like this:

      struct usbdevfs_ctrltransfer {
              __u8   bRequestType;
              __u8   bRequest;
              __u16  wValue;
              __u16  wIndex;
              __u16  wLength;
              __u32  timeout;  /* in milliseconds */
              void   *data;
      };

  The first eight bytes of this structure are the contents of the SETUP packet to be sent to the device; see the USB 2.0 specification for details. The bRequestType value is composed by combining a USB_TYPE_* value, a USB_DIR_* value, and a USB_RECIP_* value (from <linux/usb.h>). If wLength is nonzero, it describes the length of the data buffer, which is either written to the device (USB_DIR_OUT) or read from the device (USB_DIR_IN).

  At this writing, you can't transfer more than 4 KBytes of data to or from a device; usbfs has a limit, and some host controller drivers have a limit. (That's not usually a problem.) Also there's no way to say it's not OK to get a short read back from the device.

USBDEVFS_RESET

  Does a USB level device reset. The ioctl parameter is ignored. After the reset, this rebinds all device interfaces. File modification time is not updated by this request.

  Avoid using this call until some usbcore bugs get fixed, since it does not fully synchronize device, interface, and driver (not just usbfs) state.

USBDEVFS_SETINTERFACE

  Sets the alternate setting for an interface. The ioctl parameter is a pointer to a structure like this:

      struct usbdevfs_setinterface {
              unsigned int  interface;
              unsigned int  altsetting;
      };

  File modification time is not updated by this request.

  Those struct members are from some interface descriptor applying to the current configuration. The interface number is the bInterfaceNumber value, and the altsetting number is the bAlternateSetting value. (This resets each endpoint in the interface.)

USBDEVFS_SETCONFIGURATION

  Issues the `usb_set_configuration` call for the device. The parameter is an integer holding the number of a configuration (bConfigurationValue from descriptor). File modification time is not updated by this request.

  Avoid using this call until some usbcore bugs get fixed, since it does not fully synchronize device, interface, and driver (not just usbfs) state.

#### Asynchronous I/O Support

As mentioned above, there are situations where it may be important to initiate concurrent operations from user mode code. This is particularly important for periodic transfers (interrupt and isochronous), but it can be used for other kinds of USB requests too. In such cases, the asynchronous requests described here are essential. Rather than submitting one request and having the kernel block until it completes, the blocking is separate.

These requests are packaged into a structure that resembles the URB used by kernel device drivers. (No POSIX Async I/O support here, sorry.) It identifies the endpoint type (USBDEVFS_URB_TYPE_*), endpoint (number, masked with USB_DIR_IN as appropriate), buffer and length, and a user "context" value serving to uniquely identify each request. (It's usually a pointer to per-request data.) Flags can modify requests (not as many as supported for kernel drivers).

Each request can specify a realtime signal number (between SIGRTMIN and SIGRTMAX, inclusive) to request a signal be sent when the request completes.

When usbfs returns these urbs, the status value is updated, and the buffer may have been modified. Except for isochronous transfers, the actual_length is updated to say how many bytes were transferred; if the USBDEVFS_URB_DISABLE_SPD flag is set ("short packets are not OK"), if fewer bytes were read than were requested then you get an error report.

    struct usbdevfs_iso_packet_desc {
            unsigned int                     length;
            unsigned int                     actual_length;
            unsigned int                     status;
    };

    struct usbdevfs_urb {
            unsigned char                    type;
            unsigned char                    endpoint;
            int                              status;
            unsigned int                     flags;
            void                             *buffer;
            int                              buffer_length;
            int                              actual_length;
            int                              start_frame;
            int                              number_of_packets;
            int                              error_count;
            unsigned int                     signr;
            void                             *usercontext;
            struct usbdevfs_iso_packet_desc  iso_frame_desc[];
    };

For these asynchronous requests, the file modification time reflects when the request was initiated. This contrasts with their use with the synchronous requests, where it reflects when requests complete.

USBDEVFS_DISCARDURB

  TBS File modification time is not updated by this request.

USBDEVFS_DISCSIGNAL

  TBS File modification time is not updated by this request.

USBDEVFS_REAPURB

  TBS File modification time is not updated by this request.

USBDEVFS_REAPURBNDELAY

  TBS File modification time is not updated by this request.

USBDEVFS_SUBMITURB

  TBS