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    Subject[PATCH 3.5 41/88] serial: 8250: enable UART_BUG_NOMSR for Tegra
    Date
    3.5.7.30 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Stephen Warren <swarren@nvidia.com>

    commit 3685f19e07802ec4207b52465c408f185b66490e upstream.

    Tegra chips have 4 or 5 identical UART modules embedded. UARTs C..E have
    their MODEM-control signals tied off to a static state. However UARTs A
    and B can optionally route those signals to/from package pins, depending
    on the exact pinmux configuration.

    When these signals are not routed to package pins, false interrupts may
    trigger either temporarily, or permanently, all while not showing up in
    the IIR; it will read as NO_INT. This will eventually lead to the UART
    IRQ being disabled due to unhandled interrupts. When this happens, the
    kernel may print e.g.:

    irq 68: nobody cared (try booting with the "irqpoll" option)

    In order to prevent this, enable UART_BUG_NOMSR. This prevents
    UART_IER_MSI from being enabled, which prevents the false interrupts
    from triggering.

    In practice, this is not needed under any of the following conditions:

    * On Tegra chips after Tegra30, since the HW bug has apparently been
    fixed.

    * On UARTs C..E since their MODEM control signals are tied to the correct
    static state which doesn't trigger the issue.

    * On UARTs A..B if the MODEM control signals are routed out to package
    pins, since they will then carry valid signals.

    However, we ignore these exceptions for now, since they are only relevant
    if a board actually hooks up more than a 4-wire UART, and no currently
    supported board does this. If we ever support a board that does, we can
    refine the algorithm that enables UART_BUG_NOMSR to take those exceptions
    into account, and/or read a flag from DT/... that indicates that the
    board has hooked up and pinmux'd more than a 4-wire UART.

    Reported-by: Olof Johansson <olof@lixom.net> # autotester
    Signed-off-by: Stephen Warren <swarren@nvidia.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    [ luis: backported to 3.5:
    - file rename: 8250_core.c -> 8250.c ]
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
    ---
    drivers/tty/serial/8250/8250.c | 4 ++++
    1 file changed, 4 insertions(+)

    diff --git a/drivers/tty/serial/8250/8250.c b/drivers/tty/serial/8250/8250.c
    index d9e2594..899c2b9 100644
    --- a/drivers/tty/serial/8250/8250.c
    +++ b/drivers/tty/serial/8250/8250.c
    @@ -2615,6 +2615,10 @@ static void serial8250_config_port(struct uart_port *port, int flags)
    if (port->type == PORT_16550A && port->iotype == UPIO_AU)
    up->bugs |= UART_BUG_NOMSR;

    + /* HW bugs may trigger IRQ while IIR == NO_INT */
    + if (port->type == PORT_TEGRA)
    + up->bugs |= UART_BUG_NOMSR;
    +
    if (port->type != PORT_UNKNOWN && flags & UART_CONFIG_IRQ)
    autoconfig_irq(up);

    --
    1.8.3.2


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