LED devices

[Richard Purdie]

On Wed, 2007-05-30 at 14:59 +0100, Richard Hughes wrote:
> On Wed, 2007-05-30 at 14:31 +0100, Richard Purdie wrote:
> > To be honest, the LED device names are a lot more meaningful than
> > "batt1", "batt2" and "batt3" which really do tell you absolutely
> > nothing.
> 
> That's not the point. We shouldn't be scanning for magic tokens in the
> device name in sysfs.

Why not? I'm sure HAL can cope with this.

> > > If we have to parse the values in sysfs device names then we might as
> > > well go back to the horrors that are /proc. This is bad. One value per
> > > file, simple as that.
> > 
> > This doesn't break that rule. We just encode some information into the
> > device name. You don't need that information to control the LED and it
> > is machine extractable.
> 
> Yes, but so is all the stuff in /proc - but this broke everytime the
> interface changed. Scanning for tokens is wrong.

Entries under /sys/class/ get to define their interface. I don't see why
that interface can't include a naming convention for the devices which
is all this is.

> If you have the led device with extra parameters you can add parameters
> without breaking API, for instance, adding a color attribute would only
> be added to devices that have a colour.
>
> Similarly, if the device (like the thinkpad leds) supports hardware
> flashing, then an attribute could be added to define what duty cycle the
> flash exhibits. Maybe a silly idea, but helps explain my point.
>
> If the LED has a meaningful descriptor, then this too can be exposed,
> without breaking API, adding yet another ":" entry.

Adding another : does not break the API. Its certainly implied if not
explicitly specified that the name field is ':' delimited and you should
be ignoring any 3rd field or greater.

> Then the device would look like the other /sys/class entries and be easy
> to interface with in userspace/hal.
> 
> /sys/class/leds/power
>  brightness
>  description
>  color
>  subsystem/
>  trigger
>  uevent
> 
> > The whole idea behind /sys/class/ is you define a standard interface
> > which is exactly what the LED class does.
> 
> Using:delimited:names:in:the:title is not standard, and I've only seen
> it used in limited numeric form with SCSI devices and PCI devices, not
> freeform text descriptors.

If the interface defines it to work like that, it is a standard.

> > >  I'm surprised this hasn't been ripped to pieces on LKML already.
> > 
> > It was discussed, no objections were raised.
> 
> But what actual userspace code actually uses the led class at the
> moment?

Why does that matter?

Regards,

Richard