linux-kernel - Re: [PATCH] serial: Fix wakeup init logic to speed up startup

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Date:	Thu, 19 Jan 2012 11:51:47 -0800
From:	Simon Glass <sjg@...omium.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-serial@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] serial: Fix wakeup init logic to speed up startup

Hi Paul,

On Thu, Jan 19, 2012 at 11:42 AM, Paul E. McKenney
<paulmck@...ux.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@...k.pl>
>> Signed-off-by: Simon Glass <sjg@...omium.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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