linux-kernel - Re: [PATCH] pps : add non blocking option to PPS

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Message-Id: <20131011124720.bf38f18affa8baf0b009875a@linux-foundation.org>
Date:	Fri, 11 Oct 2013 12:47:20 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Paul Chavent <Paul.Chavent@...ra.fr>
Cc:	giometti@...eenne.com, linux-kernel@...r.kernel.org,
	Alexander Gordeev <agordeev@...hat.com>
Subject: Re: [PATCH] pps : add non blocking option to PPS_FETCH ioctl.

On Fri, 11 Oct 2013 14:40:32 +0200 Paul Chavent <Paul.Chavent@...ra.fr> wrote:

> The PPS_FETCH ioctl is blocking still the reception of a PPS
> event. But, in some case, one may immediately need the last event
> date. This patch allow to get the result of PPS_FETCH if the device
> has the O_NONBLOCK flag set.

Are the PPS ioctls actually documented anywhere? 
Documentation/pps/pps.txt is silent.

That's a shame, because it would be nice to have a formal description
of the the PPS_FETCH semantics which leads to an understanding of why
things are the way they are, how PPS_FETCH is supposed to be used, etc.

Also, the presence of such documentation would permit me to bug you for
not having updated it!  We need *some* channel for telling people about
the driver, and updates to it.  Maybe linuxpps.org has it somewhere,
but I couldn't immediately find it.

Your implementation requires that the file be opened non-blocking.  But
I'd have thought that adding a new and separate ioctl mode would be a
cleaner and more flexible implementation - that way an app which wants
both blocking and non-blocking behaviour doesn't need to open the file
twice.

Also, this is actually a non-backward-compatible change for any
application which happened to be opening the file with O_NONBLOCK! 
Hopefully there aren't any such applications...

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