linux-kernel - Re: [PATCH] LinuxPPS

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Message-ID: <20070724142050.GD4074@enneenne.com>
Date:	Tue, 24 Jul 2007 16:20:50 +0200
From:	Rodolfo Giometti <giometti@...eenne.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] LinuxPPS - definitive version

On Tue, Jul 24, 2007 at 02:49:02PM +0100, David Woodhouse wrote:

> Also 's/unknow /unknown /' (2 instances)

?? I didn't find them:

   $ grep 'unknow ' Documentation/pps/pps.txt

> Am I right in thinking that the only place it matters is within
> pps_event()? In that case, at the very least you should probably remove
> the 'volatile' from the definition of the structure, and _cast_ to
> volatile where you want it treated that way.

Ok, I see.

> But I don't see why you can't protect it with a spinlock. As long as you
> acquire that spinlock _after_ your call to getnstimeofday() what's the
> problem?

The problem is that we can have several PPS sources into a system and
all these sources will arise their IRQ line (quasi)simultaneously and
I don't wish a CPU may delay one of these IRQ handler due a spinlock
into the pps_event().

That's why I'm trying to avoid any lock into pps_event().

> I think you still haven't quite got the 32-bit vs. 64-bit compatibility
> right. Remember that on i386, the alignment of a uint64_t is only 4
> bytes, while on most other architectures it's 8 bytes. On i386, there
> will be no padding between the two consecutive 'struct pps_ktime'
> members of struct pps_kinfo and struct pps_kparams. But on most
> platforms there will be padding to ensure correct alignment.
> 
> The simple fix is probably to make the 'nsec' member a 64-bit integer
> too. Then it'll be the same for i386 and x86_64 and you won't need a
> compatibility syscall routine.

Ok. I'll add your comment too.

> In order for your handling of 'pps_source[source].info' to be safe with
> respect to pps_unregister_source(), you have to guarantee that
> pps_event() has finished -- and can't be in progress on another CPU --
> by the time your client's call to pps_unregister_source() completes. At
> first glance I think your existing clients have that right (you have
> del_timer_sync() before pps_unregister_source() in ktimer.c, for
> example). But you should make sure it's clearly documented for new
> clients.

This can be done only with locks, but it's not necessary since even if
a pps_unregister_source() runs while pps_event() executes on another
CPU the latter will write always on a valid area (even if it could be
a dummy one) and the data are not corrupted (note also that the data
will be, in any case, discarted since we are executing a
pps_unregister_source()).

> Shouldn't your PPS_CLIENT_LP and PPS_CLIENT_UART options depend on
> PARPORT and SERIAL_CORE respectively?

No. These options can be enabled but if no serial/parallel driver is
loaded no PPS source is registered.

Thanks,

Rodolfo

-- 

GNU/Linux Solutions                  e-mail:    giometti@...eenne.com
Linux Device Driver                             giometti@...dd.com
Embedded Systems                     		giometti@...ux.it
UNIX programming                     phone:     +39 349 2432127
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