Re: select() on /dev/input/eventX not level-triggered?

From: Pavel Machek
Date: Mon May 23 2011 - 16:06:38 EST

On Mon 2011-05-23 11:13:12, Dmitry Torokhov wrote:
> On Fri, May 20, 2011 at 01:50:51AM +0200, Pavel Machek wrote:
> > Hi!
> >
> > I'm debugging strange behaviour on /dev/input/eventX ... it seems that
> > select is not behaving level-triggered as it apparently should. I can
> > reproduce it when hitting windows & alt keys.
> >
> > Now, I really should rewrite it into C, first, but perhaps someone has
> > an idea?

(Ok, so we agree that select() should mark descriptor as ready as
long as data are available... right?)

> Is this with next or with mainline? In next we try not to signal that FD
> is ready unless we have full packet in the buffer...

2.6.39-rcX mainline.

> FWIW I see python indeed not reading the tail of events (btw the format
> should be 'llhhi' and the size on 64 bit arches is 24, not 16) but when
> I hacked evtest to use select and non-blockign read it all worked
> properly.

It was on x32, but I'll fix that, thanks.

Difference was I'm not using non-blocking read(). It is unusual but
should work AFAICT. Just read one packet when select shows its ready;
if there's more than one, don't loop around read, but rely on select
returning immediately.

I did verify it on strace, so it should not be python artefact.
(cesky, pictures)
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