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    Subject[PATCH 29/36] 8250: fix uninitialized FIFOs
    From: Ondrej Puzman <>

    I have found a bug in 8250.c driver which causes that 16550A uart FIFOs
    are not turned on during initialization if they are manually configured
    by setserial. UART is then working only as plain 16450 without FIFOs. On
    systems with higher interrupt latency this causes buffer overruns and
    loss of received data when using higher communication speeds.

    I'm working for a company which produces industrial computers. These
    devices typically contain high number (8 or more) of traditional 16550A
    uarts - we use TL16C554A chips, but that is not much relevant. UARTs are
    connected to the CPU by ISA bus (Celeron based devices) or LPC bus (Atom
    based devices).

    In the Linux the UARTs are using standard 8250.c driver and are
    initialized using setserial command:
    setserial /dev/ttyS4 uart 16550A port 0x3E0 irq 10 baud_base 115200

    This executes the UART initialization through serial8250_startup()
    function. At the beginning of the function up->capabilities is
    initialized from uart_config:
    up->capabilities = uart_config[up->port.type].flags;
    Please note that neither up->port.fifosize nor up->tx_loadsz is
    initialized here!!

    Later in the same function serial8250_clear_fifos() is called and
    disables FIFOs. The above comment says that they will be reenabled in
    set_termios (they won't ...)

    After serial8250_startup() the serial8250_set_termios() is called. In
    this function the following check fails because up->port.fifosize is
    zero because it is not initialized correctly.

    if (up->capabilities & UART_CAP_FIFO && up->port.fifosize > 1) {
    if (baud < 2400)
    fcr = uart_config[up->port.type].fcr;

    fcr variable remains zero and in the end the FCR register is set to zero
    which results in disabled FIFOs even if the UART type is 16550A. This is
    also true for other types of UARTs with FIFOs.

    If the UART is autoconfigured via 'setserial /dev/ttySx autoconfig' then
    port.fifosize and tx_loadsz are initialized correctly in the
    autoconfig() function and the UART is working correctly then.

    I checked the source codes and I can say that this bug is present in
    2.6.x series of kernels for a couple of years. Namely I can confirm its
    presence in, and (tested all of them on our

    I think it was not noticed before because not many people use manually
    configured non PNP UARTs on ISA/LPC bus these days. Also the data loss
    caused by buffer overruns occures only if IRQ latency is higher then
    time needed to receive one character on given communication speed.
    For example our hardware looses received characters only if the UARTs
    are connected throught LPC bus with SERIRQ (serial IRQ transport) and
    not if they are connected to ISA bus because LPC SERIRQ has higher
    interrupt latency then parallel ISA interupt lines.

    Here is the patch to correct the bug created against

    Signed-off-by: Ondrej Puzman <>
    Signed-off-by: Greg Kroah-Hartman <>
    drivers/serial/8250.c | 2 ++
    1 files changed, 2 insertions(+), 0 deletions(-)

    diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c
    index 9839199..8747215 100644
    --- a/drivers/serial/8250.c
    +++ b/drivers/serial/8250.c
    @@ -1981,6 +1981,8 @@ static int serial8250_startup(struct uart_port *port)
    unsigned char lsr, iir;
    int retval;

    + up->port.fifosize = uart_config[up->port.type].fifo_size;
    + up->tx_loadsz = uart_config[up->port.type].tx_loadsz;
    up->capabilities = uart_config[up->port.type].flags;
    up->mcr = 0;


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