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    SubjectRe: Linux and real device drivers
    Jes Sorensen wrote:
    >
    > >>>>> "Momchil" == Momchil Velikov <velco@fadata.bg> writes:
    >
    > Momchil> Alan Cox wrote:
    > >> There is the jump/return overhead. There is the stack management
    > >> overhead There is an extra cache line touched.
    >
    > Momchil> Ok, folks, I think we are talking about "udi_pio_trans" call
    > Momchil> (in UDI Physical I/O Specification) and this call is surely
    > Momchil> not meant to perform a single I/O port or MMIO
    > Momchil> read/write. Instead the UDI implementation executes a large
    > Momchil> transaction including many reads, writes, arithmetic
    > Momchil> operations, branches, etc. Now, the overhead is by no means
    > Momchil> in the call, it's in interpreting the transaction description
    > Momchil> and I'd say yes, it ought to be rather big.
    >
    > No, for Gigabit Ethernet I have to do some 5-10 read/writes per packet
    > to PCI shared memory, however these are not all one large block and
    > go to different places in the PCI shared mem area. Most of this will
    > have to be done with multiple UDI calls, unless they came up with a
    > scatter/gather type list for this .... yuck.

    For what I can understand from the spec, they have came up with
    some kind of a bytecode much more powerfull than a simple scatter/gather
    list. Hey, you can even store such "procedures" in a ROM on the board,
    and execute them no matter which processor or OS you use, something
    much similar to IEEE 1275's FCode.

    Regards,
    -velco

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