RE: 2.6.11, USB: High latency?

From: Alan Stern (stern_at_rowland.harvard.edu)
Date: 04/04/05

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    Date:	Mon, 4 Apr 2005 15:40:27 -0400 (EDT)
To: kus Kusche Klaus <kus@keba.com>

    On Mon, 4 Apr 2005, kus Kusche Klaus wrote:

    > I asked our hardware team. The hardware has two devices which are
    > in use and capable of busmaster/DMA transfers:
    > The intel e100 ethernet controller and the intel PIIX4 USB
    > controller.
    > The IDE interface is also a busmaster, but there are only PIO IDE
    > devices.
    >
    > I suspect the latter, as USB reads were running in parallel...
    > How many bytes are transferred at most by the USB controller
    > for a single request? How long may this take?

    Your questions aren't very clear. The USB controller will do intermittent
    DMA (once per millisecond) even when no USB transfers are pending, so long
    as a USB device is attached. When transfers are pending it will do as
    much DMA as required to complete the transfers. The amount of time
    depends on the amount of data to be transferred and the speed at which the
    device can provide/accept the data. For Bulk transfers the DMA activity
    is more or less continual.

    > Any experiences / opinitions / advices?

    I've had plenty of experience using USB, but none in measuring bus
    utilization. However my guess is that the controller puts a fairly large
    load on the system.

    Alan Stern

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