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-Specification and Internals for the New UHCI Driver (Whitepaper...)
-
-        brought to you by
-
-        Georg Acher, acher@in.tum.de (executive slave) (base guitar)
-        Deti Fliegl, deti@fliegl.de (executive slave) (lead voice)
-        Thomas Sailer, sailer@ife.ee.ethz.ch (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
-                http://hotswap.in.tum.de/usb
-
-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 
-powerful.
-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 ;-)
-Hm...
-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     
-
-framelist[]
-[  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 
-bus_to_virt()-calls.
-
-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.