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.
The patch looks good to me, but then again, I do not consider myself
qualified to have an opinion on the TTY layer. ;-)
Thanx, Paul
> Suggested-by: Rafael J. Wysocki <rjw@xxxxxxx>
> Signed-off-by: Simon Glass <sjg@xxxxxxxxxxxx>
> ---
> 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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