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Specification and Internals for the New UHCI Driver (Whitepaper...)
brought to you by
Georg Acher, (executive slave) (base guitar)
Deti Fliegl, (executive slave) (lead voice)
Thomas Sailer, (chief consultant) (cheer leader)
$Id: README.uhci,v 1.1 1999/12/14 14:03:02 fliegl Exp $
This document and the new uhci sources can be found on
1. General issues
1.1 Why a new UHCI driver, we already have one?!?
Correct, but its internal structure got more and more mixed up by the (still
ongoing) efforts to get isochronous transfers (ISO) to work.
Since there is an increasing need for reliable ISO-transfers (especially
for USB-audio needed by TS and for a DAB-USB-Receiver build by GA and DF),
this state was a bit unsatisfying in our opinion, so we've decided (based
on knowledge and experiences with the old UHCI driver) to start
from scratch with a new approach, much simpler but at the same time more
It is inspired by the way Win98/Win2000 handles USB requests via URBs,
but it's definitely 100% free of MS-code and doesn't crash while
unplugging an used ISO-device like Win98 ;-)
Some code for HW setup and root hub management was taken from the
original UHCI driver, but heavily modified to fit into the new code.
The invention of the basic concept, and major coding were completed in two
days (and nights) on the 16th and 17th of October 1999, now known as the
great USB-October-Revolution started by GA, DF, and TS ;-)
Since the concept is in no way UHCI dependent, we hope that it will also be
transferred to the OHCI-driver, so both drivers share a common API.
1.2. Advantages and disadvantages
+ All USB transfer types work now!
+ Asynchronous operation
+ Simple, but powerful interface (only two calls for start and cancel)
+ Easy migration to the new API, simplified by a compatibility API
+ Simple usage of ISO transfers
+ Automatic linking of requests
+ ISO transfers allow variable length for each frame and striping
+ No CPU dependent and non-portable atomic memory access, no asm()-inlines
+ Tested on x86 and Alpha
- Rewriting for ISO transfers needed
1.3. Is there some compatibility to the old API?
Yes, but only for control, bulk and interrupt transfers. We've implemented
some wrapper calls for these transfer types. The usbcore works fine with
these wrappers. For ISO there's no compatibility, because the old ISO-API
and its semantics were unnecessary complicated in our opinion.
1.4. What's really working?
As said above, CTRL and BULK already work fine even with the wrappers,
so legacy code wouldn't notice the change.
Regarding to Thomas, ISO transfers now run stable with USB audio.
INT transfers (e.g. mouse driver) work fine, too.
1.5. Are there any bugs?
No ;-)
Well, of course this implementation needs extensive testing on all available
hardware, but we believe that any fixes shouldn't harm the overall concept.
1.6. What should be done next?
A large part of the request handling seems to be identical for UHCI and
OHCI, so it would be a good idea to extract the common parts and have only
the HW specific stuff in uhci.c. Furthermore, all other USB device drivers
should need URBification, if they use isochronous or interrupt transfers.
One thing missing in the current implementation (and the old UHCI driver)
is fair queueing for BULK transfers. Since this would need (in principle)
the alteration of already constructed TD chains (to switch from depth to
breadth execution), another way has to be found. Maybe some simple
heuristics work with the same effect.
2. Internal structure and mechanisms
To get quickly familiar with the internal structures, here's a short
description how the new UHCI driver works. However, the ultimate source of
truth is only uhci.c!
2.1. Descriptor structure (QHs and TDs)
During initialization, the following skeleton is allocated in init_skel:
framespecific | common chain
[ 0 ]-----> TD --> TD -------\
[ 1 ]-----> TD --> TD --------> TD ----> QH -------> QH -------> QH ---> NULL
... TD --> TD -------/
[1023]-----> TD --> TD ------/
^^ ^^ ^^ ^^ ^^ ^^
1024 TDs for 7 TDs for 1 TD for Start of Start of End Chain
ISO INT (2-128ms) 1ms-INT CTRL Chain BULK Chain
For each CTRL or BULK transfer a new QH is allocated and the containing data
transfers are appended as (vertical) TDs. After building the whole QH with its
dangling TDs, the QH is inserted before the BULK Chain QH (for CTRL) or
before the End Chain QH (for BULK). Since only the QH->next pointers are
affected, no atomic memory operation is required. The three QHs in the
common chain are never equipped with TDs!
For ISO or INT, the TD for each frame is simply inserted into the appropriate
ISO/INT-TD-chain for the desired frame. The 7 skeleton INT-TDs are scattered
among the 1024 frames similar to the old UHCI driver.
For CTRL/BULK/ISO, the last TD in the transfer has the IOC-bit set. For INT,
every TD (there is only one...) has the IOC-bit set.
Besides the data for the UHCI controller (2 or 4 32bit words), the descriptors
are double-linked through the .vertical and .horizontal elements in the
SW data of the descriptor (using the double-linked list structures and
operations), but SW-linking occurs only in closed domains, i.e. for each of
the 1024 ISO-chains and the 8 INT-chains there is a closed cycle. This
simplifies all insertions and unlinking operations and avoids costly
2.2. URB structure and linking to QH/TDs
During assembly of the QH and TDs of the requested action, these descriptors
are stored in urb->urb_list, so the allocated QH/TD descriptors are bound to
this URB.
If the assembly was successful and the descriptors were added to the HW chain,
the corresponding URB is inserted into a global URB list for this controller.
This list stores all pending URBs.
2.3. Interrupt processing
Since UHCI provides no means to directly detect completed transactions, the
following is done in each UHCI interrupt (uhci_interrupt()):
For each URB in the pending queue (process_urb()), the ACTIVE-flag of the
associated TDs are processed (depending on the transfer type
process_{transfer|interrupt|iso}()). If the TDs are not active anymore,
they indicate the completion of the transaction and the status is calculated.
Inactive QH/TDs are removed from the HW chain (since the host controller
already removed the TDs from the QH, no atomic access is needed) and
eventually the URB is marked as completed (OK or errors) and removed from the
pending queue. Then the next linked URB is submitted. After (or immediately
before) that, the completion handler is called.
2.4. Unlinking URBs
First, all QH/TDs stored in the URB are unlinked from the HW chain.
To ensure that the host controller really left a vertical TD chain, we
wait for one frame. After that, the TDs are physically destroyed.
2.5. URB linking and the consequences
Since URBs can be linked and the corresponding submit_urb is called in
the UHCI-interrupt, all work associated with URB/QH/TD assembly has to be
interrupt save. This forces kmalloc to use GFP_ATOMIC in the interrupt.