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-rw-r--r--Documentation/usb/ehci.txt42
1 files changed, 29 insertions, 13 deletions
diff --git a/Documentation/usb/ehci.txt b/Documentation/usb/ehci.txt
index 160bd6c3ab7b..31f650e7c1b4 100644
--- a/Documentation/usb/ehci.txt
+++ b/Documentation/usb/ehci.txt
@@ -1,3 +1,7 @@
+===========
+EHCI driver
+===========
+
27-Dec-2002
The EHCI driver is used to talk to high speed USB 2.0 devices using
@@ -40,7 +44,8 @@ APIs exposed to USB device drivers.
<dbrownell@users.sourceforge.net>
-FUNCTIONALITY
+Functionality
+=============
This driver is regularly tested on x86 hardware, and has also been
used on PPC hardware so big/little endianness issues should be gone.
@@ -48,6 +53,7 @@ It's believed to do all the right PCI magic so that I/O works even on
systems with interesting DMA mapping issues.
Transfer Types
+--------------
At this writing the driver should comfortably handle all control, bulk,
and interrupt transfers, including requests to USB 1.1 devices through
@@ -63,6 +69,7 @@ since EHCI represents these with a different data structure. So for now,
most USB audio and video devices can't be connected to high speed buses.
Driver Behavior
+---------------
Transfers of all types can be queued. This means that control transfers
from a driver on one interface (or through usbfs) won't interfere with
@@ -83,14 +90,15 @@ limits on the number of periodic transactions that can be scheduled,
and prevent use of polling intervals of less than one frame.
-USE BY
+Use by
+======
Assuming you have an EHCI controller (on a PCI card or motherboard)
-and have compiled this driver as a module, load this like:
+and have compiled this driver as a module, load this like::
# modprobe ehci-hcd
-and remove it by:
+and remove it by::
# rmmod ehci-hcd
@@ -112,13 +120,16 @@ If you're using this driver on a 2.5 kernel, and you've enabled USB
debugging support, you'll see three files in the "sysfs" directory for
any EHCI controller:
- "async" dumps the asynchronous schedule, used for control
+ "async"
+ dumps the asynchronous schedule, used for control
and bulk transfers. Shows each active qh and the qtds
pending, usually one qtd per urb. (Look at it with
usb-storage doing disk I/O; watch the request queues!)
- "periodic" dumps the periodic schedule, used for interrupt
+ "periodic"
+ dumps the periodic schedule, used for interrupt
and isochronous transfers. Doesn't show qtds.
- "registers" show controller register state, and
+ "registers"
+ show controller register state, and
The contents of those files can help identify driver problems.
@@ -136,7 +147,8 @@ transaction translators are in use; some drivers have been seen to behave
badly when they see different faults than OHCI or UHCI report.
-PERFORMANCE
+Performance
+===========
USB 2.0 throughput is gated by two main factors: how fast the host
controller can process requests, and how fast devices can respond to
@@ -156,6 +168,7 @@ hardware and device driver software allow it. Periodic transfer modes
approach the quoted 480 MBit/sec transfer rate.
Hardware Performance
+--------------------
At this writing, individual USB 2.0 devices tend to max out at around
20 MByte/sec transfer rates. This is of course subject to change;
@@ -183,6 +196,7 @@ you issue a control or bulk request you can often expect to learn that
it completed in less than 250 usec (depending on transfer size).
Software Performance
+--------------------
To get even 20 MByte/sec transfer rates, Linux-USB device drivers will
need to keep the EHCI queue full. That means issuing large requests,
@@ -206,9 +220,11 @@ mapping (which might apply an IOMMU) and IRQ reduction, all of which will
help make high speed transfers run as fast as they can.
-TBD: Interrupt and ISO transfer performance issues. Those periodic
-transfers are fully scheduled, so the main issue is likely to be how
-to trigger "high bandwidth" modes.
+TBD:
+ Interrupt and ISO transfer performance issues. Those periodic
+ transfers are fully scheduled, so the main issue is likely to be how
+ to trigger "high bandwidth" modes.
-TBD: More than standard 80% periodic bandwidth allocation is possible
-through sysfs uframe_periodic_max parameter. Describe that.
+TBD:
+ More than standard 80% periodic bandwidth allocation is possible
+ through sysfs uframe_periodic_max parameter. Describe that.