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Diffstat (limited to 'Documentation/DocBook/usb.tmpl')
| -rw-r--r-- | Documentation/DocBook/usb.tmpl | 123 |
1 files changed, 62 insertions, 61 deletions
diff --git a/Documentation/DocBook/usb.tmpl b/Documentation/DocBook/usb.tmpl index 320af25..3608472 100644 --- a/Documentation/DocBook/usb.tmpl +++ b/Documentation/DocBook/usb.tmpl @@ -43,59 +43,52 @@ <para>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 at the + devices. USB uses a tree structure, with the host as the root (the system's master), hubs as interior nodes, and - peripheral devices as leaves (and slaves). + 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". </para> - <para>That master/slave asymmetry was designed in part for - 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 the downstream type are the squarish type - (or they are built in to the peripheral). - Software doesn't need to deal with distributed autoconfiguration - since the pre-designated master node manages all that. - At the electrical level, bus protocol overhead is reduced by - eliminating arbitration and moving scheduling into host software. + <para>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. </para> - <para>USB 1.0 was announced in January 1996, and was revised + <para>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, including high speed - transfers and transaction translating hubs (used for USB 1.1 + 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). </para> - <para>USB support was added to Linux early in the 2.2 kernel series - shortly before the 2.3 development forked off. Updates - from 2.3 were regularly folded back into 2.2 releases, bringing - new features such as <filename>/sbin/hotplug</filename> support, - more drivers, and more robustness. - The 2.5 kernel series continued such improvements, and also - worked on USB 2.0 support, - higher performance, - better consistency between host controller drivers, - API simplification (to make bugs less likely), - and providing internal "kerneldoc" documentation. + <para>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 <filename>/sbin/hotplug</filename> 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. </para> <para>Linux can run inside USB devices as well as on the hosts that control the devices. - Because the Linux 2.x USB support evolved to support mass market - platforms such as Apple Macintosh or PC-compatible systems, - it didn't address design concerns for those types of USB systems. - So it can't be used inside mass-market PDAs, or other peripherals. - USB device drivers running inside those Linux peripherals + But USB device drivers running inside those peripherals don't do the same things as the ones running inside hosts, - and so they've been given a different name: - they're called <emphasis>gadget drivers</emphasis>. - This document does not present gadget drivers. + so they've been given a different name: + <emphasis>gadget drivers</emphasis>. + This document does not cover gadget drivers. </para> </chapter> @@ -103,17 +96,14 @@ <chapter id="host"> <title>USB Host-Side API Model</title> - <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 - driver frameworks such as block, character, or network devices; - and drivers that are <emphasis>part of the core</emphasis>, - which are involved in managing a USB bus. - Such core drivers include the <emphasis>hub</emphasis> driver, - which manages trees of USB devices, and several different kinds - of <emphasis>host controller driver (HCD)</emphasis>, + <para>Host-side drivers for USB devices talk to the "usbcore" APIs. + There are two. One is intended for + <emphasis>general-purpose</emphasis> drivers (exposed through + driver frameworks), and the other is for drivers that are + <emphasis>part of the core</emphasis>. + Such core drivers include the <emphasis>hub</emphasis> driver + (which manages trees of USB devices) and several different kinds + of <emphasis>host controller drivers</emphasis>, which control individual busses. </para> @@ -122,21 +112,21 @@ <itemizedlist> - <listitem><para>USB supports four kinds of data transfer - (control, bulk, interrupt, and isochronous). Two transfer - types use bandwidth as it's available (control and bulk), - while the other two types of transfer (interrupt and isochronous) + <listitem><para>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. </para></listitem> <listitem><para>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 that might be used. + 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. </para></listitem> - <listitem><para>Configurations have one or more "interface", each + <listitem><para>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.</para> @@ -162,7 +152,7 @@ </para></listitem> <listitem><para>The Linux USB API supports synchronous calls for - control and bulk messaging. + control and bulk messages. It also supports asynchnous calls for all kinds of data transfer, using request structures called "URBs" (USB Request Blocks). </para></listitem> @@ -463,14 +453,25 @@ 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>This file, in combination with the poll() system call, can + also be used to detect when devices are added or removed: +<programlisting>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.) */ +}</programlisting> + 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. </para> - </sect1> <sect1> |
