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