linux-kernel - Re: [PATCH 1/1] LinuxPPS: Pulse per Second support for Linux

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Message-ID: <45DA6354.3000305@zytor.com>
Date:	Mon, 19 Feb 2007 18:56:20 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Rodolfo Giometti <giometti@...eenne.com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] LinuxPPS: Pulse per Second support for Linux

Rodolfo Giometti wrote:
> 
> Please read the following consideratios before sending to /dev/null!
> :)
> 
> RFC considerations
> ------------------
> 
> While implementing a PPS API as RFC 2783 defines and using an embedded
> CPU GPIO-Pin as physical link to the signal, I encountered a deeper
> problem:
> 
>    At startup it needs a file descriptor as argument for the function
>    time_pps_create().
> 
> This implies that the source has a /dev/... entry. This assumption is
> ok for the serial and parallel port, where you can do something
> usefull beside(!) the gathering of timestamps as it is the central
> task for a PPS-API. But this assumption does not work for a single
> purpose GPIO line. In this case even basic file-related functionality
> (like read() and write()) makes no sense at all and should not be a
> precondition for the use of a PPS-API.
> 

It's not a precondition for a file descriptor, either.  There are plenty 
of ioctl-only device drivers in existence.

Furthermore, a file descriptor doesn't imply a device entry.  Consider 
pipe(2), for example.

As far as the kernel is concerned, a file handle is a nice, uniform 
system for providing communication between the kernel and user space. 
It doesn't matter if one can read() or write() on it; it's perfectly 
normal to support only a subset of the normal operations.

	-hpa
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