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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
+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.