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SubjectRe: [PATCH 2.6.8.1 0/2] leds: new class for led devices
On Fri, Sep 03, 2004 at 07:09:59PM +0000, John Lenz wrote:
> > > The led class does not really inforce any policy, it just passes this
> > > number along to the actual driver that is registered. So say you had
> > > a led that could be red, green, or both red and green at the same time
> > > (not sure how that would work hardware wise, but ok).
> > See Netwinders. They have a bi-colour LED which has independent red
> > and green LEDs in the same package. When they're both on it's yellow.
> >
> > It's _VERY_ useful to see the green LED flashing and know that the
> > headless machine is running, or that the red LED being on means that
> > either it hasn't booted a kernel yet or the system has successfully
> > shut down.
> >
> > It means I don't have to fsck around with monitor cables before pulling
> > the power.
> >
> > And no, doing that from userspace won't work because userspace is dead
> > _before_ the system has finished shutting down (see drive cache
> > flushing on shutdown.)
> >
> > It's also _VERY_ useful to know whether the kernel has managed to get
> > far enough through the boot that the heartbeat LED is flashing but
> > maybe not sufficiently to bring up the serial console as well -
> > again another thing that a userspace implementation will never be
> > able to support.
>
> Hey I agree! I use the led as a debugging tool on the Sharp Zaurus.
> Before I had the framebuffer or the serial port working, being able to see
> the led told me something was working :) This patch isn't really intended
> for debugging support, so perhaps some other interface could be added.
>
> On the other hand, perhaps we can just do a #ifdef LEDS_DEBUGGING that
> would toggle the led on a timer.

You missed the point. The above scenarios I was describing are _not_
debugging.

--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 PCMCIA - http://pcmcia.arm.linux.org.uk/
2.6 Serial core
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