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USB: update intro of documentation
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It does no good to mention The 2.4 kernel series and neglect
USB 3.x and XHCI. Also with type C and micro/mini USB we better
not talk about the shape of connectors.

Signed-off-by: Oliver Neukum <[email protected]>
Acked-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
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oneukum authored and Jonathan Corbet committed Oct 27, 2016
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68 changes: 30 additions & 38 deletions Documentation/DocBook/usb.tmpl
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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".
a few USB 3.0 (5 GBit/s) or USB 3.1 (10 GBit/s) and some legacy
USB 2.0 (480 MBit/s) busses just in case.
</para>

<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
mistake upstream and downstream or it does not matter with a type C
plug
(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
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).
</para>

<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.
series and have been developing it further since then. Besides support
for each new generation of USB, various host controllers gained support,
new drivers for peripherals have been added and advanced features for latency
measurement and improved power management introduced.
</para>

<para>Linux can run inside USB devices as well as on
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<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 which might be used.
Devices are supposed to be capable of operating at lower than their top
speeds and may provide a BOS descriptor showing the lowest speed they
remain fully operational at.
</para></listitem>

<listitem><para>From USB 3.0 on configurations have one or more "functions", which
provide a common functionality and are grouped together for purposes
of power management.
</para></listitem>

<listitem><para>Configurations have one or more "interfaces", each
<listitem><para>Configurations or functions 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>
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Think of them as "interface drivers", though you
may not see many devices where the distinction is important.
<emphasis>Most USB devices are simple, with only one configuration,
one interface, and one alternate setting.</emphasis>
one function, one interface, and one alternate setting.</emphasis>
</para></listitem>

<listitem><para>Interfaces have one or more "endpoints", each of
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<para>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
the USB 3.0 specification, available online from www.usb.org at
no cost, as well as class or device specifications.
</para>

<para>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,
API. In practice, that's becoming mostly true,
but there are still differences that crop up especially with
fault handling. Different controllers don't necessarily report
fault handling on the less common controllers.
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
thei1r 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.)
</para>

</chapter>
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<para>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.
handles 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
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<para>These APIs are only for use by host controller drivers,
most of which implement standard register interfaces such as
EHCI, OHCI, or UHCI.
XHCI, 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).
XHCI was designed with USB 3.0. It continues to shift support
for functionality into hardware.
</para>

<para>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.
is also a simulator and a virtual host controller to pipe
USB over the network.
</para>

<para>The same basic APIs are available to drivers for all
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<structname>struct usb_bus</structname> is a rather thin
layer that became available in the 2.2 kernels, while
<structname>struct usb_hcd</structname> is a more featureful
layer (available in later 2.4 kernels and in 2.5) that
layer that
lets HCDs share common code, to shrink driver size
and significantly reduce hcd-specific behaviors.
</para>
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