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Commit b2d7ebeb authored by David Brownell's avatar David Brownell Committed by Greg Kroah-Hartman
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[PATCH] USB: kerneldoc for usbfs 

So far as I know, "usbfs" was never documented ... so
here's a patch with some text I've had sitting around,
merging it into the other USB kerneldoc.

It should be accurate, down to the warnings about why
not to use several of the calls, though cross-review
with the code would be good too.  A notable omission
is any details on the async APIs.
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Documentation/DocBook/usb.tmpl
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......@@ -100,7 +100,8 @@
<chapter id="host">
<title>USB Host-Side API Model</title>
<para>Host-side drivers for USB devices talk to the "usbcore" APIs.
<para>Within the kernel,
host-side drivers for USB devices talk to the "usbcore" APIs.
There are two types of public "usbcore" APIs, targetted at two different
layers of USB driver. Those are
<emphasis>general purpose</emphasis> drivers, exposed through
......@@ -289,6 +290,684 @@
!Edrivers/usb/core/buffer.c
</chapter>
<chapter>
<title>The USB Filesystem (usbfs)</title>
<para>This chapter presents the Linux <emphasis>usbfs</emphasis>.
You may prefer to avoid 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
<ulink url="http://libusb.sourceforge.net">libusb</ulink>
for C/C++, and
<ulink url="http://jUSB.sourceforge.net">jUSB</ulink> for Java.
</para>
<note><title>Unfinished</title>
<para>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.
</para>
</note>
<para>Configure usbfs into Linux kernels by enabling the
<emphasis>USB filesystem</emphasis> option (CONFIG_USB_DEVICEFS),
and you get basic support for user mode USB device drivers.
Until relatively recently it was often (confusingly) called
<emphasis>usbdevfs</emphasis> although it wasn't solving what
<emphasis>devfs</emphasis> was.
Every USB device will appear in usbfs, regardless of whether or
not it has a kernel driver; but only devices with kernel drivers
show up in devfs.
</para>
<sect1>
<title>What files are in "usbfs"?</title>
<para>Conventionally mounted at
<filename>/proc/bus/usb</filename>, usbfs
features include:
<itemizedlist>
<listitem><para><filename>/proc/bus/usb/devices</filename>
... 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.
</para></listitem>
<listitem><para><filename>/proc/bus/usb/BBB/DDD</filename>
... 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.)
</para></listitem>
</itemizedlist>
</para>
<para> 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.
<emphasis>Don't even think of saving these in application
configuration files.</emphasis>
Stable identifiers are available, for user mode applications
that want to use them. HID and networking devices expose
these IDs.
</para>
</sect1>
<sect1>
<title>Mounting and Access Control</title>
<para>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
(<filename>/proc/bus/BBB/DDD</filename>) although anyone may read
the <filename>devices</filename>
or <filename>drivers</filename> files.
I/O requests to the device also need the CAP_SYS_RAWIO capability,
</para>
<para>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 <emphasis>devmode=0666</emphasis> option)
so the driver can start whenever it's needed.
</para>
<para>The mount options for usbfs, usable in /etc/fstab or
in command line invocations of <emphasis>mount</emphasis>, are:
<variablelist>
<varlistentry>
<term><emphasis>busgid</emphasis>=NNNNN</term>
<listitem><para>Controls the GID used for the
/proc/bus/usb/BBB
directories. (Default: 0)</para></listitem></varlistentry>
<varlistentry><term><emphasis>busmode</emphasis>=MMM</term>
<listitem><para>Controls the file mode used for the
/proc/bus/usb/BBB
directories. (Default: 0555)
</para></listitem></varlistentry>
<varlistentry><term><emphasis>busuid</emphasis>=NNNNN</term>
<listitem><para>Controls the UID used for the
/proc/bus/usb/BBB
directories. (Default: 0)</para></listitem></varlistentry>
<varlistentry><term><emphasis>devgid</emphasis>=NNNNN</term>
<listitem><para>Controls the GID used for the
/proc/bus/usb/BBB/DDD
files. (Default: 0)</para></listitem></varlistentry>
<varlistentry><term><emphasis>devmode</emphasis>=MMM</term>
<listitem><para>Controls the file mode used for the
/proc/bus/usb/BBB/DDD
files. (Default: 0644)</para></listitem></varlistentry>
<varlistentry><term><emphasis>devuid</emphasis>=NNNNN</term>
<listitem><para>Controls the UID used for the
/proc/bus/usb/BBB/DDD
files. (Default: 0)</para></listitem></varlistentry>
<varlistentry><term><emphasis>listgid</emphasis>=NNNNN</term>
<listitem><para>Controls the GID used for the
/proc/bus/usb/devices and drivers files.
(Default: 0)</para></listitem></varlistentry>
<varlistentry><term><emphasis>listmode</emphasis>=MMM</term>
<listitem><para>Controls the file mode used for the
/proc/bus/usb/devices and drivers files.
(Default: 0444)</para></listitem></varlistentry>
<varlistentry><term><emphasis>listuid</emphasis>=NNNNN</term>
<listitem><para>Controls the UID used for the
/proc/bus/usb/devices and drivers files.
(Default: 0)</para></listitem></varlistentry>
</variablelist>
</para>
<para>Note that many Linux distributions hard-wire the mount options
for usbfs in their init scripts, such as
<filename>/etc/rc.d/rc.sysinit</filename>,
rather than making it easy to set this per-system
policy in <filename>/etc/fstab</filename>.
</para>
</sect1>
<sect1>
<title>/proc/bus/usb/devices</title>
<para>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
<filename>Documentation/usb/proc_usb_info.txt</filename>
file in your Linux kernel sources.
</para>
<para>Otherwise the main use for this file from programs
is to poll() it to get notifications of usb devices
as they're plugged or unplugged.
To see what changed, you'd need to read the file and
compare "before" and "after" contents, scan the filesystem,
or see its hotplug event.
</para>
</sect1>
<sect1>
<title>/proc/bus/usb/BBB/DDD</title>
<para>Use these files in one of these basic ways:
</para>
<para><emphasis>They can be read,</emphasis>
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.
</para>
<para><emphasis>Perform USB operations</emphasis> using
<emphasis>ioctl()</emphasis> 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 <emphasis>half duplex</emphasis> protocols,
but otherwise you'd use asynchronous i/o requests.
</para>
</sect1>
<sect1>
<title>Life Cycle of User Mode Drivers</title>
<para>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
<filename>/sbin/hotplug</filename> 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 <function>read()</function> 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.
</para>
<para>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.
</para>
<para>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.)
</para>
<para>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.
<emphasis>Bulk</emphasis> transfers are easiest to use,
but only their sibling <emphasis>interrupt</emphasis> transfers
work with low speed devices.
Both interrupt and <emphasis>isochronous</emphasis> 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.
</para>
<para>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.
</para>
</sect1>
<sect1><title>The ioctl() Requests</title>
<para>To use these ioctls, you need to include the following
headers in your userspace program:
<programlisting>#include &lt;linux/usb.h&gt;
#include &lt;linux/usbdevice_fs.h&gt;
#include &lt;asm/byteorder.h&gt;</programlisting>
The standard USB device model requests, from "Chapter 9" of
the USB 2.0 specification, are automatically included from
the <filename>&lt;linux/usb_ch9.h&gt;</filename> header.
</para>
<para>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
<filename>Documentation/usb/error-codes.txt</filename>
in your kernel sources.)
</para>
<para>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
<emphasis>configuration</emphasis> that affects things
like power consumption and basic functionality.
The endpoints are part of USB <emphasis>interfaces</emphasis>,
which may have <emphasis>altsettings</emphasis>
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.
</para>
<sect2>
<title>Management/Status Requests</title>
<para>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.
</para>
<variablelist>
<varlistentry><term>USBDEVFS_CLAIMINTERFACE</term>
<listitem><para>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).
</para><para>
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.
</para><para>
This claim will be released by a RELEASEINTERFACE ioctl,
or by closing the file descriptor.
File modification time is not updated by this request.
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_CONNECTINFO</term>
<listitem><para>Says whether the device is lowspeed.
The ioctl parameter points to a structure like this:
<programlisting>struct usbdevfs_connectinfo {
unsigned int devnum;
unsigned char slow;
}; </programlisting>
File modification time is not updated by this request.
</para><para>
<emphasis>You can't tell whether a "not slow"
device is connected at high speed (480 MBit/sec)
or just full speed (12 MBit/sec).</emphasis>
You should know the devnum value already,
it's the DDD value of the device file name.
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_GETDRIVER</term>
<listitem><para>Returns the name of the kernel driver
bound to a given interface (a string). Parameter
is a pointer to this structure, which is modified:
<programlisting>struct usbdevfs_getdriver {
unsigned int interface;
char driver[USBDEVFS_MAXDRIVERNAME + 1];
};</programlisting>
File modification time is not updated by this request.
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_IOCTL</term>
<listitem><para>Passes a request from userspace through
to a kernel driver that has an ioctl entry in the
<emphasis>struct usb_driver</emphasis> it registered.
<programlisting>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, &amp;wrapper);
} </programlisting>
File modification time is not updated by this request.
</para><para>
This request lets kernel drivers talk to user mode code
through filesystem operations even when they don't create
a charactor 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.
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_RELEASEINTERFACE</term>
<listitem><para>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.
</para><warning><para>
<emphasis>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. </emphasis>
</para></warning></listitem></varlistentry>
<varlistentry><term>USBDEVFS_RESETEP</term>
<listitem><para>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.
</para><warning><para>
<emphasis>Avoid using this request.
It should probably be removed.</emphasis>
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.
</para></warning></listitem></varlistentry>
</variablelist>
</sect2>
<sect2>
<title>Synchronous I/O Support</title>
<para>Synchronous requests involve the kernel blocking
until 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.
</para>
<variablelist>
<varlistentry><term>USBDEVFS_BULK</term>
<listitem><para>Issues a bulk read or write request to the
device.
The ioctl parameter is a pointer to this structure:
<programlisting>struct usbdevfs_bulktransfer {
unsigned int ep;
unsigned int len;
unsigned int timeout; /* in milliseconds */
void *data;
};</programlisting>
</para><para>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.
<emphasis>FIXME say how read length is returned,
and how short reads are handled.</emphasis>.
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_CLEAR_HALT</term>
<listitem><para>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.
</para><para>
Use this on bulk or interrupt endpoints which have
stalled, returning <emphasis>-EPIPE</emphasis> 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.
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_CONTROL</term>
<listitem><para>Issues a control request to the device.
The ioctl parameter points to a structure like this:
<programlisting>struct usbdevfs_ctrltransfer {
__u8 bRequestType;
__u8 bRequest;
__u16 wValue;
__u16 wIndex;
__u16 wLength;
__u32 timeout; /* in milliseconds */
void *data;
};</programlisting>
</para><para>
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
<emphasis>&lt;linux/usb.h&gt;</emphasis>).
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).
</para><para>
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.)
<emphasis>Also</emphasis> there's no way to say it's
not OK to get a short read back from the device.
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_RESET</term>
<listitem><para>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.
</para><warning><para>
<emphasis>Avoid using this call</emphasis>
until some usbcore bugs get fixed,
since it does not fully synchronize device, interface,
and driver (not just usbfs) state.
</para></warning></listitem></varlistentry>
<varlistentry><term>USBDEVFS_SETINTERFACE</term>
<listitem><para>Sets the alternate setting for an
interface. The ioctl parameter is a pointer to a
structure like this:
<programlisting>struct usbdevfs_setinterface {
unsigned int interface;
unsigned int altsetting;
}; </programlisting>
File modification time is not updated by this request.
</para><para>
Those struct members are from some interface descriptor
applying to the 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.)
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_SETCONFIGURATION</term>
<listitem><para>Issues the
<function>usb_set_configuration</function> 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.
</para><warning><para>
<emphasis>Avoid using this call</emphasis>
until some usbcore bugs get fixed,
since it does not fully synchronize device, interface,
and driver (not just usbfs) state.
</para></warning></listitem></varlistentry>
</variablelist>
</sect2>
<sect2>
<title>Asynchronous I/O Support</title>
<para>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.
</para>
<para>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).
</para>
<para>Each request can specify a realtime signal number
(between SIGRTMIN and SIGRTMAX, inclusive) to request a
signal be sent when the request completes.
</para>
<para>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.
</para>
<programlisting>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[];
};</programlisting>
<para> 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.
</para>
<variablelist>
<varlistentry><term>USBDEVFS_DISCARDURB</term>
<listitem><para>
<emphasis>TBS</emphasis>
File modification time is not updated by this request.
</para><para>
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_DISCSIGNAL</term>
<listitem><para>
<emphasis>TBS</emphasis>
File modification time is not updated by this request.
</para><para>
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_REAPURB</term>
<listitem><para>
<emphasis>TBS</emphasis>
File modification time is not updated by this request.
</para><para>
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_REAPURBNDELAY</term>
<listitem><para>
<emphasis>TBS</emphasis>
File modification time is not updated by this request.
</para><para>
</para></listitem></varlistentry>
<varlistentry><term>USBDEVFS_SUBMITURB</term>
<listitem><para>
<emphasis>TBS</emphasis>
</para><para>
</para></listitem></varlistentry>
</variablelist>
</sect2>
</sect1>
</chapter>
</book>
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