Re: [PARPORT] DMA and the parallel port

From: Tim Waugh (
Date: Sat May 03 2003 - 04:48:15 EDT

  • Next message: Bret Comstock Waldow: "Re: [PARPORT] figuring out an ECP port problem"

    On Fri, May 02, 2003 at 11:34:23PM -0700, Gora Mohanty wrote:

    > Is it really true that the speed difference is negligible?

    Well, it's only really true for bulk transfers in a defined IEEE 1284
    transfer mode like ECP, to be fair. For custom protocols it will be
    much slower in user-space, but for IEEE 1284 modes: it depends. When
    user-space write()s a large amount, the kernel is doing the transfer
    protocol stuff anyway, so in that sense the difference should be
    negligible for that situation.

    Basically I imagine it depends how much of the logic is in user-space.

    > Also, does anyone know if there is a limit to the number of parallel
    > ports/PCI parallel port cards that can be supported by Linux on a x86 PC?
    > A friend claims that only a max. of 4 can be supported, though he could
    > not explain why. I also see that my PC has only /dev/parport0 through
    > /dev/parport3.

    There's a #define you can tweak in inlude/linux/parport.h (when
    recompiling the parport modules). PARPORT_MAX is currently set at 16
    in the kernel I'm using.


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