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    SubjectRe: Disabling DMA with ICH10?
    On Tue, 29 Sep 2009 09:54:16 +0200
    "Lennart Baruschka" <FunFlyer@gmx.net> wrote:

    > Hi,
    >
    > On Mon, 2009-09-28 at 23:40 +0100, Alan Cox wrote:
    > > > 2. My system uses an ICH10 chipset, the harddrive is connected to
    > > > the Intel SATA controller. Is there a way to disable DMA and
    > > > switch back to PIO? I tried compiling with libata support and
    > > > without SCSI support, but the kernel is unable to mount root (no
    > > > matter if /dev/sda1 or /dev/hda1), then.
    > >
    > > If you disable DMA you will make your performance and latency worse
    > > not better as the PIO transfers will stall the bus and thus the
    > > processor.
    >
    > I thought that PIO transfers (which I understand to be
    > write32()/read32()'s) unlike DMA transfers could be interrupted by an
    > high-priority interrupt. Is that wrong?

    a single PIO instruction will NOT be interrupted by an interrupt.
    And can easily take several microseconds (remember: 8Mhz bus emulation)

    while DMA is going on, the CPU on the other hand is just happy
    executing instructions, fetching stuff from memory etc etc.


    > Actually, that's what I do - except for locking the page, yet. I do
    > need to access the PCI bus in real time, though. So I wonder what
    > happens when the RT CPU is getting data from the PCI device, doing
    > some calculations on it and then writing back some data to the device,
    > __while at the same time__ another (non-RT) CPU starts a DMA
    > transfer. I figured the DMA would block the PCI bus, having my
    > interrupt wait for it to finish. That's why I'm trying to avoid DMA.

    the PCI bus is time sliced, with typical transfer sizes being like 256
    byte bursts...



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