Re: Serial driver hangs

From: Paul Fulghum (paulkf_at_microgate.com)
Date: 09/30/04

  • Next message: Andrea Arcangeli: "Re: mlock(1)" 
    To: Roland Caßebohm <roland.cassebohm@VisionSystems.de>
Date:	Thu, 30 Sep 2004 14:09:58 -0500

    On Thu, 2004-09-30 at 11:16, Roland Caßebohm wrote:
    > Maybe I can use the functions rs_throttle() and
    > rs_unthrottle(). In rs_unthrottle I could reenable the RX
    > interrupt. So I don't need to add a function in "struct
    > tty_driver". I only need to set the flag TTY_THROTTLED if I
    > disable the interrupt.

    The question is when to unthrottle:

    Doing it in N_TTY read_chan() does not mean a
    flip buffer is available.

    Doing it in flush_to_ldisc() could unthrottle
    when the ldisc receive buffer is still full.

    It is a problem of using a single mechanism to
    throttle the receiver in response to
    two seperate triggers (flip buffer state and
    ldisc receive buffer state).

    > Maybe TTY_DONT_FLIP is really don't needed anymore.

    The only use I see for TTY_DONT_FLIP is
    delaying transfers of data from a flip buffer
    to the N_TTY receive buffer via ldisc->receive_buf()
    while N_TTY read_chan() is returning data
    from the N_TTY receive buffer to the user app.
    This behavior maximizes free space in the
    N_TTY receive buffer before sending the next
    flip buffer to ldisc->receive_buf().

    N_TTY ldisc receive buffer access is
    protected by tty->read_lock, so not using
    TTY_DONT_FLIP wont corrupt this buffer.
    However, if there is not enough room in the
    N_TTY receive buffer when ldisc->receive_buf()
    is called then data is lost.

    > I think to be save and fast maybe one way could be, if the
    > flip buffer is full it should be flipped but not processed
    > with tty->ldisc.receive_buf() in the interrupt routine.
    > flush_to_ldisc() has then always to look at both flip buffers
    > and process them.
    > If the second flip buffer is still not clean, if the interrupt
    > routine needs to flip it, it has to stop the flow and disable
    > the receive interrupt.
    > unthrottle() could then reenable the interrupt.

    The gaping hole in the flip buffer scheme is
    flush_to_ldisc() can be called from hard IRQ
    context while ldisc->receive_buf() is running.

    This can flip buffers while ldisc->receive_buf() is
    still reading from one of the flip buffers.
    That corrupts data if the device ISR overwrites
    the buffer before ldisc->receive_buf() finishes
    reading from it.

    A mechanism is required to prevent flipping
    buffers while ldisc->receive_buf() is running.
    TTY_DONT_FLIP would work, but is already in
    use for the purpose described above :-)

    I understand the plan is to eliminate or replace
    the flip buffer scheme. I doubt patches tinkering
    with the current scheme will be accepted. 

    -- 
Paul Fulghum
paulkf@microgate.com
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  • Next message: Andrea Arcangeli: "Re: mlock(1)"