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diff --git a/Documentation/usb/uhci.txt b/Documentation/usb/uhci.txt new file mode 100644 index 0000000..2f25952 --- /dev/null +++ b/Documentation/usb/uhci.txt @@ -0,0 +1,165 @@ +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. |
