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EHCI: improved logic for isochronous scheduling
This patch (as1608) reworks the logic used by ehci-hcd for scheduling isochronous transfers. Now the modular calculations are all based on a window that starts at the last frame scanned for isochronous completions. No transfer descriptors for any earlier frames can possibly remain on the schedule, so there can be no confusion from schedule wrap-around. This removes the need for a "slop" region of arbitrary size. There's no need to check for URBs that are longer than the schedule length. With the old code they could throw things off by wrapping around and appearing to end in the near future rather than the distant future. Now such confusion isn't possible, and the existing test for submissions that extend too far into the future will also catch those that exceed the schedule length. (But there still has to be an initial test to handle the case where the schedule already extends as far into the future as possible.) Delays caused by IRQ latency won't confuse the algorithm unless they are ridiculously long (over 250 ms); they will merely reduce how far into the future new transfers can be scheduled. A few people have reported problems caused by delays of 50 ms or so. Now instead of failing completely, isochronous transfers will experience a brief glitch and then continue normally. (Whether this is truly a good thing is debatable. A latency as large as 50 ms generally indicates a bug is present, and complete failure of audio or video transfers draws people's attention pretty vividly. Making the transfers more robust also makes it easier for such bugs to remain undetected.) Finally, ehci->next_frame is renamed to ehci->last_iso_frame, because that better describes what it is: the last frame to have been scanned for isochronous completions. Signed-off-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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