linux-kernel - Re: LinuxPPS kernel interface

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Message-ID: <20090208015633.GA15809@csclub.uwaterloo.ca>
Date:	Sat, 7 Feb 2009 20:56:33 -0500
From:	lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
To:	George Spelvin <linux@...izon.com>
Cc:	giometti@...eenne.com, linux-kernel@...r.kernel.org,
	LinuxPPS@...enneenne.com
Subject: Re: LinuxPPS kernel interface

On Sat, Feb 07, 2009 at 07:49:14PM -0500, George Spelvin wrote:
> (Be aware that this is crossposted to linux-kernel and LinuxPPS.
> The latter mailing list rejects mail from non-subscribers, so
> the discussion on linux-kernel is likely to be more complete.)
> 
> LinuxPPS is an implementation of the PPS API from RFC 2783.
> That's a function-call interface, and it's up to the implementation
> to determine how to partition responsibility between the kernel and
> the userspace library.
> 
> I just posted to the LinuxPPS maining list a series of patches which
> change the semantics of the LinuxPPS kernel interface so most of the
> mode bits are per-fd rather than global.  You can have one user listening
> for assert events and the other listening for clear events.
> 
> So far, nothing has changed the interface for any single user; the
> only difference is that multiple users now interact less.
> 
> One thing I got working is that POLLIN now properly waits until a new
> PPS event is available.
> 
> That makes it possible to implement the (required) time_pps_fetch()
> function without passing the timeout to the PPS_FETCH ioctl.  Instead,
> time_pps_fetch() can be implemented as a call to poll(2) followed by an
> immediate ioctl.
> 
> However, other kernel interface cleanups seem possible.  Here's my list
> of possible kernel interface changes.  I'd like some design review and
> comments on the ideas.
> (Note that some of them are mutually exclusive!)
> 
> - Eliminate the timeout argument and the whole struct pps_fdata thing.
>   Do the timeout with ppoll, and then get the last event.
> 
> - Change PPS_FETCH to never wait.  Instead, it always immediately returns
>   the most recently captures PPS events.  If you want to wait, use poll.
> 
> - Move the PPS_OFFSET feature to user space.  That requires making
>   a pps_handle_t a pointer to a structure containing the offsets.
> 
> - Move the PPS_CAPTURE mode bits to use space.  Rather than telling
>   the kernel the mode of the current fd, just have the two different
>   edges reported by different poll bits (POLLPRI and POLLRDBAND?) which
>   userspace can wait for whenever it wishes.  (POLLIN would get both,
>   of course.)
> 
> - Add a read(2) interface.  Suggestions for the format are appreciated.
>   I imagine an ASCII interface:
>   assert <sequence> <seconds>.<fraction>\n
>   clear <sequence> <seconds>.<fraction>\n
>   But maybe something else?
> 
> - Replace PPS_FETCH with the above interface?  Or is the dual binary/ASCII
>   conversion too annoying?
> 
> - Add a clock type specification to the kernel interface?  Or is
>   CLOCK_REALTIME always okay?
> 
> - I'd like to add support for serial port signals other than DCD.
>   The 8250 hardware treats CTS and DSR just the same, and one edge of RI
>   can be trapped.  Any idea how to do that?  Just instantiate multiple
>   PPS devices?  Should there be a better device naming convention?
> 
> - Think about the ECHO flags.  The RFC leaves it implementation-defined
>   which line is used for output, and people seem to use the RTS line.
>   Bascially, the interrupt handler does a TIOCMBIS/TIOCMBIC ioctl to
>   toggle the RTS line.  Is this okay?  How should it interact with CRTSCTS?

Have you looked at the PPS code Rodolfo Giometti has been working on and
has been working hard to get in to the kernel?  It works very well, and
I think probably does everything (or at least most of what) you
mentioned.  I have been using it for a couple of years now.

I would not start working on yet another implementation.

Search for mailing list archives for LinusPPS core.  The latest version
was posted quite recently (less than a month ago).

-- 
Len Sorensen
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