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    firewire: add isochronous multichannel reception · 872e330e
    Stefan Richter 提交于
    This adds the DMA context programming and userspace ABI for multichannel
    reception, i.e. for listening on multiple channel numbers by means of a
    single DMA context.
    
    The use case is reception of more streams than there are IR DMA units
    offered by the link layer.  This is already implemented by the older
    ohci1394 + ieee1394 + raw1394 stack.  And as discussed recently on
    linux1394-devel, this feature is occasionally used in practice.
    
    The big drawbacks of this mode are that buffer layout and interrupt
    generation necessarily differ from single-channel reception:  Headers
    and trailers are not stripped from packets, packets are not aligned with
    buffer chunks, interrupts are per buffer chunk, not per packet.
    
    These drawbacks also cause a rather hefty code footprint to support this
    rarely used OHCI-1394 feature.  (367 lines added, among them 94 lines of
    added userspace ABI documentation.)
    
    This implementation enforces that a multichannel reception context may
    only listen to channels to which no single-channel context on the same
    link layer is presently listening to.  OHCI-1394 would allow to overlay
    single-channel contexts by the multi-channel context, but this would be
    a departure from the present first-come-first-served policy of IR
    context creation.
    
    The implementation is heavily based on an earlier one by Jay Fenlason.
    Thanks Jay.
    Signed-off-by: NStefan Richter <stefanr@s5r6.in-berlin.de>
    872e330e
firewire.h 12.4 KB