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    SubjectRe: [PATCH] serial: Fix wakeup init logic to speed up startup
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    Hi Paul,

    On Thu, Jan 19, 2012 at 11:42 AM, Paul E. McKenney
    <paulmck@linux.vnet.ibm.com> wrote:
    > On Thu, Jan 19, 2012 at 11:28:56AM -0800, Simon Glass wrote:
    >> The synchronize_rcu() call resulting from making every serial driver
    >> wake-up capable (commit b3b708fa) slows boot down on my Tegra2x system
    >> (with CONFIG_PREEMPT disabled).
    >>
    >> But this is avoidable since it is the device_set_wakeup_enable() and then
    >> subsequence disable which causes the delay. We might as well just make
    >> the device wakeup capable but not actually enable it for wakeup until
    >> needed.
    >>
    >> Effectively the current code does this:
    >>
    >>       device_set_wakeup_capable(dev, 1);
    >>       device_set_wakeup_enable(dev, 1);
    >>       device_set_wakeup_enable(dev, 0);
    >>
    >> We can just drop the last two lines.
    >>
    >> Before this change my boot log says:
    >> [    0.227062] Serial: 8250/16550 driver, 4 ports, IRQ sharing disabled
    >> [    0.702928] serial8250.0: ttyS0 at MMIO 0x70006040 (irq = 69) is a Tegra
    >>
    >> after:
    >> [    0.227264] Serial: 8250/16550 driver, 4 ports, IRQ sharing disabled
    >> [    0.227983] serial8250.0: ttyS0 at MMIO 0x70006040 (irq = 69) is a Tegra
    >>
    >> for saving of 450ms.
    >
    > You have multiple CPUs running at this point, correct?  Before that
    > second CPU starts up, synchronize_rcu() is a no-op.

    Yes that's right, although I didn't get different behavior with 'nosmp'.

    >
    > The patch looks good to me, but then again, I do not consider myself
    > qualified to have an opinion on the TTY layer.  ;-)

    Thanks, me neither :-)

    Regards,
    Simon

    >
    >                                                        Thanx, Paul
    >
    >> Suggested-by: Rafael J. Wysocki <rjw@sisk.pl>
    >> Signed-off-by: Simon Glass <sjg@chromium.org>
    >> ---
    >>  drivers/tty/serial/serial_core.c |    6 +++---
    >>  1 files changed, 3 insertions(+), 3 deletions(-)
    >>
    >> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
    >> index c7bf31a..1305618 100644
    >> --- a/drivers/tty/serial/serial_core.c
    >> +++ b/drivers/tty/serial/serial_core.c
    >> @@ -2348,11 +2348,11 @@ int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport)
    >>        */
    >>       tty_dev = tty_register_device(drv->tty_driver, uport->line, uport->dev);
    >>       if (likely(!IS_ERR(tty_dev))) {
    >> -             device_init_wakeup(tty_dev, 1);
    >> -             device_set_wakeup_enable(tty_dev, 0);
    >> -     } else
    >> +             device_set_wakeup_capable(tty_dev, 1);
    >> +     } else {
    >>               printk(KERN_ERR "Cannot register tty device on line %d\n",
    >>                      uport->line);
    >> +     }
    >>
    >>       /*
    >>        * Ensure UPF_DEAD is not set.
    >> --
    >> 1.7.7.3
    >>
    >
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