Re: evdev types

[Vojtech P. <vo...@su...>]

On Thu, Apr 18, 2002 at 05:23:51PM +1000, Brad Hards wrote:
> On Wed, 17 Apr 2002 23:18, Vojtech Pavlik wrote:
> > On Wed, Apr 17, 2002 at 09:56:05PM +1000, Brad Hards wrote:
> <snip>
> > > Ah, I think I have it now. For
> > >
> > > ioctl(a, EVIOCGBIT(b, c), d)
> > >
> > > a is the file descriptor for the event device you want
> > >
> > > b is the type of descriptor bitfield to return
> > > 	0 for the type of descriptors that are used
> > > 	EV_KEY for the key bitfield
> > > 	EV_REL for the relative axes bitfield
> > > 	EV_ABS for the absolute axes bitfield
> > > 	EV_MSC for the miscellaneous bitfield (not used yet?)
> > > 	EV_LED for the bright lights bitfield
> > > 	EV_SND for the loud noises bitfield
> > > 	EV_REP for the repeat function bitfield (not used yet?)
> > > 	EV_FF for the force feedback bitfield
> > >
> > > c is the maximum number of bits to return
> > > 	If the number of bits available is greater, then you get a warning
> > > message
> >
> > Bytes.
> Actually, it is more complex than that. The code all assumes that there is a 
> long on the other end.
> So my sample code for reading LED values (simplified here for shortness, let 
> me know if you want it all):
> <programlisting>
>   uint8_t led_bitmask[LED_MAX/8 + 1];
> ...
>   ioctl(fd, EVIOCGBIT(EV_LED, sizeof(led_bitmask)), led_bitmask)
> </programlisting>
> 
> is hitting the truncation check, because I'm only providing two bytes, and it 
> wants to return four (long, on ix86). Makes a mess in my syslog, and yet it 
> is reasonable from an application point of view, since the led_bitmask 
> doesn't really need to be an array - I could have just malloc'd it, and it is 
> big enough to hold all valid bit settings.
> 
> Why use long arrays for the bitfields? Why not byte arrays? This seems like 
> the easiest way to clean up your concerns with 32/64 split systems, and also 
> to clean up this.

Because set_bit/test_bit and friends all operate on longs.

Regarding the two bytes/four bytes stuff we can:

1) Remove the warning and all will work OK

or

2) Don't warn if the valid bits actually fit

And for /proc and hotplug I think we can specify that we'll always use
32-bit numbers (we currently use longs (%lx), where the size varies
between 32 and 64-bit kernels on the same system).

> > > OK, my only question now is "how do you get the descriptor bitfield for
> > > EV_RST?"
> >
> > There is none. There are no bits for EV_RST, it's only one event in its
> > own class.

> OK. This also applies to EV_REP?

Yes.

> Thanks again for all your help.

-- 
Vojtech Pavlik
SuSE Labs