Based on kernel version 2.6.26. Page generated on 2008-07-16 21:13 EST.
1 Specification and Internals for the New UHCI Driver (Whitepaper...) 2 3 brought to you by 4 5 Georg Acher, acher[AT]in.tum[DOT]de (executive slave) (base guitar) 6 Deti Fliegl, deti[AT]fliegl[DOT]de (executive slave) (lead voice) 7 Thomas Sailer, sailer[AT]ife.ee.ethz[DOT]ch (chief consultant) (cheer leader) 8 9 $Id: README.uhci,v 1.1 1999/12/14 14:03:02 fliegl Exp $ 10 11 This document and the new uhci sources can be found on 12 http://hotswap.in.tum.de/usb 13 14 1. General issues 15 16 1.1 Why a new UHCI driver, we already have one?!? 17 18 Correct, but its internal structure got more and more mixed up by the (still 19 ongoing) efforts to get isochronous transfers (ISO) to work. 20 Since there is an increasing need for reliable ISO-transfers (especially 21 for USB-audio needed by TS and for a DAB-USB-Receiver build by GA and DF), 22 this state was a bit unsatisfying in our opinion, so we've decided (based 23 on knowledge and experiences with the old UHCI driver) to start 24 from scratch with a new approach, much simpler but at the same time more 25 powerful. 26 It is inspired by the way Win98/Win2000 handles USB requests via URBs, 27 but it's definitely 100% free of MS-code and doesn't crash while 28 unplugging an used ISO-device like Win98 ;-) 29 Some code for HW setup and root hub management was taken from the 30 original UHCI driver, but heavily modified to fit into the new code. 31 The invention of the basic concept, and major coding were completed in two 32 days (and nights) on the 16th and 17th of October 1999, now known as the 33 great USB-October-Revolution started by GA, DF, and TS ;-) 34 35 Since the concept is in no way UHCI dependent, we hope that it will also be 36 transferred to the OHCI-driver, so both drivers share a common API. 37 38 1.2. Advantages and disadvantages 39 40 + All USB transfer types work now! 41 + Asynchronous operation 42 + Simple, but powerful interface (only two calls for start and cancel) 43 + Easy migration to the new API, simplified by a compatibility API 44 + Simple usage of ISO transfers 45 + Automatic linking of requests 46 + ISO transfers allow variable length for each frame and striping 47 + No CPU dependent and non-portable atomic memory access, no asm()-inlines 48 + Tested on x86 and Alpha 49 50 - Rewriting for ISO transfers needed 51 52 1.3. Is there some compatibility to the old API? 53 54 Yes, but only for control, bulk and interrupt transfers. We've implemented 55 some wrapper calls for these transfer types. The usbcore works fine with 56 these wrappers. For ISO there's no compatibility, because the old ISO-API 57 and its semantics were unnecessary complicated in our opinion. 58 59 1.4. What's really working? 60 61 As said above, CTRL and BULK already work fine even with the wrappers, 62 so legacy code wouldn't notice the change. 63 Regarding to Thomas, ISO transfers now run stable with USB audio. 64 INT transfers (e.g. mouse driver) work fine, too. 65 66 1.5. Are there any bugs? 67 68 No ;-) 69 Hm... 70 Well, of course this implementation needs extensive testing on all available 71 hardware, but we believe that any fixes shouldn't harm the overall concept. 72 73 1.6. What should be done next? 74 75 A large part of the request handling seems to be identical for UHCI and 76 OHCI, so it would be a good idea to extract the common parts and have only 77 the HW specific stuff in uhci.c. Furthermore, all other USB device drivers 78 should need URBification, if they use isochronous or interrupt transfers. 79 One thing missing in the current implementation (and the old UHCI driver) 80 is fair queueing for BULK transfers. Since this would need (in principle) 81 the alteration of already constructed TD chains (to switch from depth to 82 breadth execution), another way has to be found. Maybe some simple 83 heuristics work with the same effect. 84 85 --------------------------------------------------------------------------- 86 87 2. Internal structure and mechanisms 88 89 To get quickly familiar with the internal structures, here's a short 90 description how the new UHCI driver works. However, the ultimate source of 91 truth is only uhci.c! 92 93 2.1. Descriptor structure (QHs and TDs) 94 95 During initialization, the following skeleton is allocated in init_skel: 96 97 framespecific | common chain 98 99 framelist[] 100 [ 0 ]-----> TD --> TD -------\ 101 [ 1 ]-----> TD --> TD --------> TD ----> QH -------> QH -------> QH ---> NULL 102 ... TD --> TD -------/ 103 [1023]-----> TD --> TD ------/ 104 105 ^^ ^^ ^^ ^^ ^^ ^^ 106 1024 TDs for 7 TDs for 1 TD for Start of Start of End Chain 107 ISO INT (2-128ms) 1ms-INT CTRL Chain BULK Chain 108 109 For each CTRL or BULK transfer a new QH is allocated and the containing data 110 transfers are appended as (vertical) TDs. After building the whole QH with its 111 dangling TDs, the QH is inserted before the BULK Chain QH (for CTRL) or 112 before the End Chain QH (for BULK). Since only the QH->next pointers are 113 affected, no atomic memory operation is required. The three QHs in the 114 common chain are never equipped with TDs! 115 116 For ISO or INT, the TD for each frame is simply inserted into the appropriate 117 ISO/INT-TD-chain for the desired frame. The 7 skeleton INT-TDs are scattered 118 among the 1024 frames similar to the old UHCI driver. 119 120 For CTRL/BULK/ISO, the last TD in the transfer has the IOC-bit set. For INT, 121 every TD (there is only one...) has the IOC-bit set. 122 123 Besides the data for the UHCI controller (2 or 4 32bit words), the descriptors 124 are double-linked through the .vertical and .horizontal elements in the 125 SW data of the descriptor (using the double-linked list structures and 126 operations), but SW-linking occurs only in closed domains, i.e. for each of 127 the 1024 ISO-chains and the 8 INT-chains there is a closed cycle. This 128 simplifies all insertions and unlinking operations and avoids costly 129 bus_to_virt()-calls. 130 131 2.2. URB structure and linking to QH/TDs 132 133 During assembly of the QH and TDs of the requested action, these descriptors 134 are stored in urb->urb_list, so the allocated QH/TD descriptors are bound to 135 this URB. 136 If the assembly was successful and the descriptors were added to the HW chain, 137 the corresponding URB is inserted into a global URB list for this controller. 138 This list stores all pending URBs. 139 140 2.3. Interrupt processing 141 142 Since UHCI provides no means to directly detect completed transactions, the 143 following is done in each UHCI interrupt (uhci_interrupt()): 144 145 For each URB in the pending queue (process_urb()), the ACTIVE-flag of the 146 associated TDs are processed (depending on the transfer type 147 process_{transfer|interrupt|iso}()). If the TDs are not active anymore, 148 they indicate the completion of the transaction and the status is calculated. 149 Inactive QH/TDs are removed from the HW chain (since the host controller 150 already removed the TDs from the QH, no atomic access is needed) and 151 eventually the URB is marked as completed (OK or errors) and removed from the 152 pending queue. Then the next linked URB is submitted. After (or immediately 153 before) that, the completion handler is called. 154 155 2.4. Unlinking URBs 156 157 First, all QH/TDs stored in the URB are unlinked from the HW chain. 158 To ensure that the host controller really left a vertical TD chain, we 159 wait for one frame. After that, the TDs are physically destroyed. 160 161 2.5. URB linking and the consequences 162 163 Since URBs can be linked and the corresponding submit_urb is called in 164 the UHCI-interrupt, all work associated with URB/QH/TD assembly has to be 165 interrupt save. This forces kmalloc to use GFP_ATOMIC in the interrupt.