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    SubjectRe: patch: serial 16550C autoflow
    On Wed, Sep 23, 1998 at 06:42:58PM -0400, Theodore Y. Ts'o wrote:
    > I've found that hardware receive autoflow based on the FIFO flowing
    > generally isn't a great idea. First of all, if the system can't handle
    > the serial interrupt within 8 character times, there's something pretty
    > seriously wrong, and automatic flow control merely hides the problem.
    > If you are in this bad situation, yanking the flow control signals up
    > and down rapidly as the FIFO fills and empties is also not good for
    > performance, because there is often a delay before a device starts
    > sending after it stops sending due to a flow control signal.

    Possibly. But at the data rates mentioned (400kbaud), 8 character times
    is 20us, which is pushing it for Linux on a PC at the moment. And if
    there's another interrupt being serviced, goodbye data.

    I wonder why they don't just put a bigger FIFO on the serial chip.

    Still agree with the rest of your post.

    -- Jamie




    >
    > Also, many devices aren't can't stop instantly, either. That's why
    > Linux as a 1024 byte internal buffer, and will start signalling the flow
    > control signal when there 128 bytes left. I want to give plenty of time
    > for the flow control signal to happen.
    >
    > As you yourself point out in the source code, this can cause some data
    > transfer lockups.
    >
    > + * *** Currently only implemented for the 16550C. The 16650=
    >
    > + * can do autoflow too, but it uses different registers.
    >
    > Not true. The National Semiconductor 16550 can not do automatic flow
    > control.
    >
    > In any case, I'll look at supporting the 16550C, but only if someone can
    > identify the manufacturer and I can get a hold of some formal datasheets
    > describing it. I've done some web searching, and nothing's shown up as
    > far as an manufacturer is concerned.
    >
    > - Ted
    >
    > Please read the FAQ at http://www.tux.org/lkml/
    >
    > ------------------------------

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