Messages in this thread | | | Date | Wed, 18 Jan 2012 13:08:13 -0800 | Subject | Re: [PATCH 3/3] serial: 8250: Add a wakeup_capable module param | From | Simon Glass <> |
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[+cc Rafael J. Wysocki <rjw@sisk.pl> who I think wrote the wakeup.c code]
Hi Alan, Paul,
On Tue, Jan 17, 2012 at 8:17 PM, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > On Tue, Jan 17, 2012 at 08:10:36PM +0000, Alan Cox wrote: >> On Tue, 17 Jan 2012 10:56:03 -0800 >> Simon Glass <sjg@chromium.org> wrote: >> >> > Since serial_core now does not make serial ports wake-up capable by >> > default, add a parameter to support this feature in the 8250 UART. >> > This is the only UART where I think this feature is useful. >> >> NAK >> >> Things should just work for users. Magic parameters is not an >> improvement. If its a performance problem someone needs to fix the rcu >> sync overhead or stop using rcu on that path.
OK fair enough, I agree. Every level I move down the source tree affects more people though.
> > I must say that I lack context here, even after looking at the patch, > but the synchronize_rcu_expedited() primitives can be used if the latency > of synchronize_rcu() is too large. >
Let me provide a bit of context. The serial_core code seems to be the only place in the kernel that does this:
device_init_wakeup(tty_dev, 1); device_set_wakeup_enable(tty_dev, 0);
The first call makes the device wakeup capable and enables wakeup, The second call disabled wakeup.
The code that removes the wakeup source looks like this:
void wakeup_source_remove(struct wakeup_source *ws) { if (WARN_ON(!ws)) return;
spin_lock_irq(&events_lock); list_del_rcu(&ws->entry); spin_unlock_irq(&events_lock); synchronize_rcu(); }
The sync is there because we are about to destroy the actual ws structure (in wakeup_source_destroy()). I wonder if it should be in wakeup_source_destroy() but that wouldn't help me anyway.
synchronize_rcu_expedited() is a bit faster but not really fast enough. Anyway surely people will complain if I put this in the wakeup code - it will affect all wakeup users. It seems to me that the right solution is to avoid enabling and then immediately disabling wakeup.
I assume we can't and shouldn't change device_init_wakeup() . We could add a call like device_init_wakeup_disabled() which makes the device wakeup capable but does not actually enable it. Does that work?
Regards, Simon
> Thanx, Paul > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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