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    Subject[PATCH v11 18/19] serial: imx: clean up imx_poll_get_char()
    Date
    From: Dirk Behme <dirk.behme@de.bosch.com>

    Looking at the get_poll_char() function of the 8250.c serial driver,
    we learn:

    * poll_get_char() doesn't have to save/disable/restore the interrupt
    registers. No interrupt handling is needed in this function at all.
    Remove it.

    * Don't block in case there is no data available. So instead blocking
    in the do {} while loop, just return with NO_POLL_CHAR, immediately .

    Additionally, while the i.MX6 register URXD[7-0] contain the RX_DATA,
    the upper bits of this register (URXD[15-10]) might contain some
    control flags. To ensure that these are not returned with the data
    read, just mask out URXD[7-0].

    These changes fix the 'hang' working with kdb:

    $ echo ttymxc3 > /sys/module/kgdboc/parameters/kgdboc
    $ echo g >/proc/sysrq-trigger
    [0]kdb> help
    ...
    <hang>

    Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
    Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Cc: Jiri Slaby <jslaby@suse.cz>
    Cc: linux-serial@vger.kernel.org
    ---
    drivers/tty/serial/imx.c | 29 ++++-------------------------
    1 file changed, 4 insertions(+), 25 deletions(-)

    diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
    index 044e86d..983668a 100644
    --- a/drivers/tty/serial/imx.c
    +++ b/drivers/tty/serial/imx.c
    @@ -80,6 +80,7 @@
    #define URXD_FRMERR (1<<12)
    #define URXD_BRK (1<<11)
    #define URXD_PRERR (1<<10)
    +#define URXD_RX_DATA (0xFF<<0)
    #define UCR1_ADEN (1<<15) /* Auto detect interrupt */
    #define UCR1_ADBR (1<<14) /* Auto detect baud rate */
    #define UCR1_TRDYEN (1<<13) /* Transmitter ready interrupt enable */
    @@ -1506,32 +1507,10 @@ imx_verify_port(struct uart_port *port, struct serial_struct *ser)
    #if defined(CONFIG_CONSOLE_POLL)
    static int imx_poll_get_char(struct uart_port *port)
    {
    - struct imx_port_ucrs old_ucr;
    - unsigned int status;
    - unsigned char c;
    -
    - /* save control registers */
    - imx_port_ucrs_save(port, &old_ucr);
    -
    - /* disable interrupts */
    - writel(UCR1_UARTEN, port->membase + UCR1);
    - writel(old_ucr.ucr2 & ~(UCR2_ATEN | UCR2_RTSEN | UCR2_ESCI),
    - port->membase + UCR2);
    - writel(old_ucr.ucr3 & ~(UCR3_DCD | UCR3_RI | UCR3_DTREN),
    - port->membase + UCR3);
    -
    - /* poll */
    - do {
    - status = readl(port->membase + USR2);
    - } while (~status & USR2_RDR);
    -
    - /* read */
    - c = readl(port->membase + URXD0);
    -
    - /* restore control registers */
    - imx_port_ucrs_restore(port, &old_ucr);
    + if (!(readl(port->membase + USR2) & USR2_RDR))
    + return NO_POLL_CHAR;

    - return c;
    + return readl(port->membase + URXD0) & URXD_RX_DATA;
    }

    static void imx_poll_put_char(struct uart_port *port, unsigned char c)
    --
    1.9.3


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