Skip to content
  • Alan Cox's avatar
    [PATCH] TTY layer buffering revamp · 33f0f88f
    Alan Cox authored
    The API and code have been through various bits of initial review by
    serial driver people but they definitely need to live somewhere for a
    while so the unconverted drivers can get knocked into shape, existing
    drivers that have been updated can be better tuned and bugs whacked out.
    This replaces the tty flip buffers with kmalloc objects in rings. In the
    normal situation for an IRQ driven serial port at typical speeds the
    behaviour is pretty much the same, two buffers end up allocated and the
    kernel cycles between them as before.
    When there are delays or at high speed we now behave far better as the
    buffer pool can grow a bit rather than lose characters. This also means
    that we can operate at higher speeds reliably.
    For drivers that receive characters in blocks (DMA based, USB and
    especially virtualisation) the layer allows a lot of driver specific
    code that works around the tty layer with private secondary queues to be
    removed. The IBM folks need this sort of layer, the smart serial port
    people do, the virtualisers do (because a virtualised tty typically
    operates at infinite speed rather than emulating 9600 baud).
    Finally many drivers had invalid and unsafe attempts to avoid buffer
    overflows by directly invoking tty methods extracted out of the innards
    of work queue structs. These are no longer needed and all go away. That
    fixes various random hangs with serial ports on overflow.
    The other change in here is to optimise the receive_room path that is
    used by some callers. It turns out that only one ldisc uses receive room
    except asa constant and it updates it far far less than the value is
    read. We thus make it a variable not a function call.
    I expect the code to contain bugs due to the size alone but I'll be
    watching and squashing them and feeding out new patches as it goes.
    Because the buffers now dynamically expand you should only run out of
    buffering when the kernel runs out of memory for real.  That means a lot of
    the horrible hacks high performance drivers used to do just aren't needed any
    tty_insert_flip_char is an old API and continues to work as before, as does
    tty_flip_buffer_push() [this is why many drivers dont need modification].  It
    does now also return the number of chars inserted
    There are also
    tty_buffer_request_room(tty, len)
    which asks for a buffer block of the length requested and returns the space
    found.  This improves efficiency with hardware that knows how much to
    and tty_insert_flip_string_flags(tty, str, flags, len)
    to insert a string of characters and flags
    For a smart interface the usual code is
        len = tty_request_buffer_room(tty, amount_hardware_says);
        tty_insert_flip_string(tty, buffer_from_card, len);
    More description!
    At the moment tty buffers are attached directly to the tty.  This is causing a
    lot of the problems related to tty layer locking, also problems at high speed
    and also with bursty data (such as occurs in virtualised environments)
    I'm working on ripping out the flip buffers and replacing them with a pool of
    dynamically allocated buffers.  This allows both for old style "byte I/O"
    devices and also helps virtualisation and smart devices where large blocks of
    data suddenely materialise and need storing.
    So far so good.  Lots of drivers reference tty->flip.*.  Several of them also
    call directly and unsafely into function pointers it provides.  This will all
    break.  Most drivers can use tty_insert_flip_char which can be kept as an API
    but others need more.
    At the moment I've added the following interfaces, if people think more will
    be needed now is a good time to say
     int tty_buffer_request_room(tty, size)
    Try and ensure at least size bytes are available, returns actual room (may be
    zero).  At the moment it just uses the flipbuf space but that will change.
    Repeated calls without characters being added are not cumulative.  (ie if you
    call it with 1, 1, 1, and then 4 you'll have four characters of space.  The
    other functions will also try and grow buffers in future but this will be a
    more efficient way when you know block sizes.
     int tty_insert_flip_char(tty, ch, flag)
    As before insert a character if there is room.  Now returns 1 for success, 0
    for failure.
     int tty_insert_flip_string(tty, str, len)
    Insert a block of non error characters.  Returns the number inserted.
     int tty_prepare_flip_string(tty, strptr, len)
    Adjust the buffer to allow len characters to be added.  Returns a buffer
    pointer in strptr and the length available.  This allows for hardware that
    needs to use functions like insl or mencpy_fromio.
    Signed-off-by: default avatarAlan Cox <>
    Cc: Paul Fulghum <>
    Signed-off-by: default avatarHirokazu Takata <>
    Signed-off-by: default avatarSerge Hallyn <>
    Signed-off-by: default avatarJeff Dike <>
    Signed-off-by: default avatarJohn Hawkes <>
    Signed-off-by: default avatarMartin Schwidefsky <>
    Signed-off-by: default avatarAdrian Bunk <>
    Signed-off-by: default avatarAndrew Morton <>
    Signed-off-by: default avatarLinus Torvalds <>