• Stefan Richter's avatar
    firewire: ohci: fix JMicron JMB38x IT context discovery · 100ceb66
    Stefan Richter authored
    Reported by Clifford and Craig for JMicron OHCI-1394 + SDHCI combo
    controllers:  Often or even most of the time, the controller is
    initialized with the message "added OHCI v1.10 device as card 0, 4 IR +
    0 IT contexts, quirks 0x10".  With 0 isochronous transmit DMA contexts
    (IT contexts), applications like audio output are impossible.
    However, OHCI-1394 demands that at least 4 IT contexts are implemented
    by the link layer controller, and indeed JMicron JMB38x do implement
    four of them.  Only their IsoXmitIntMask register is unreliable at early
    With my own JMB381 single function controller I found:
      - I can reproduce the problem with a lower probability than Craig's.
      - If I put a loop around the section which clears and reads
        IsoXmitIntMask, then either the first or the second attempt will
        return the correct initial mask of 0x0000000f.  I never encountered
        a case of needing more than a second attempt.
      - Consequently, if I put a dummy reg_read(...IsoXmitIntMaskSet)
        before the first write, the subsequent read will return the correct
      - If I merely ignore a wrong read result and force the known real
        result, later isochronous transmit DMA usage works just fine.
    So let's just fix this chip bug up by the latter method.  Tested with
    JMB381 on kernel 3.13 and 4.3.
    Since OHCI-1394 generally requires 4 IT contexts at a minium, this
    workaround is simply applied whenever the initial read of IsoXmitIntMask
    returns 0, regardless whether it's a JMicron chip or not.  I never heard
    of this issue together with any other chip though.
    I am not 100% sure that this fix works on the OHCI-1394 part of JMB380
    and JMB388 combo controllers exactly the same as on the JMB381 single-
    function controller, but so far I haven't had a chance to let an owner
    of a combo chip run a patched kernel.
    Strangely enough, IsoRecvIntMask is always reported correctly, even
    though it is probed right before IsoXmitIntMask.
    Reported-by: Clifford Dunn
    Reported-by: default avatarCraig Moore <craig.moore@qenos.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarStefan Richter <stefanr@s5r6.in-berlin.de>
ohci.c 106 KB