linux/Documentation/usb/ohci.txt
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   123-Aug-2002
   2
   3The "ohci-hcd" driver is a USB Host Controller Driver (HCD) that is derived
   4from the "usb-ohci" driver from the 2.4 kernel series.  The "usb-ohci" code
   5was written primarily by Roman Weissgaerber <weissg@vienna.at> but with
   6contributions from many others (read its copyright/licencing header).
   7
   8It supports the "Open Host Controller Interface" (OHCI), which standardizes
   9hardware register protocols used to talk to USB 1.1 host controllers.  As
  10compared to the earlier "Universal Host Controller Interface" (UHCI) from
  11Intel, it pushes more intelligence into the hardware.  USB 1.1 controllers
  12from vendors other than Intel and VIA generally use OHCI.
  13
  14Changes since the 2.4 kernel include
  15
  16        - improved robustness; bugfixes; and less overhead
  17        - supports the updated and simplified usbcore APIs
  18        - interrupt transfers can be larger, and can be queued
  19        - less code, by using the upper level "hcd" framework
  20        - supports some non-PCI implementations of OHCI
  21        - ... more
  22
  23The "ohci-hcd" driver handles all USB 1.1 transfer types.  Transfers of all
  24types can be queued.  That was also true in "usb-ohci", except for interrupt
  25transfers.  Previously, using periods of one frame would risk data loss due
  26to overhead in IRQ processing.  When interrupt transfers are queued, those
  27risks can be minimized by making sure the hardware always has transfers to
  28work on while the OS is getting around to the relevant IRQ processing.
  29
  30- David Brownell
  31  <dbrownell@users.sourceforge.net>
  32
  33
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