Experimental Physics and
Industrial Control System

Benjamin Franksen <[email protected]> · Wed, 19 Jun 2013 18:10:32 +0200

Hi

again sorry for long delay; you are not forgotten!

On Wednesday, June 12, 2013 12:38:40 J. Lewis Muir wrote:
> On 6/12/13 11:10 AM, Benjamin Franksen wrote:
> > On Monday, June 10, 2013 17:22:12 Carl Lionberger wrote:
> >> Looking at the code in seq_ca.c and seq_if.c, it seems that if a
> >> synchronous pvGet ever fails to get a callback from channel access, the
> >> "get completion timeout" message is set in the channel metadata and all
> >> subsequent attempts to do pvGets on that channel will fail as in the
> >> first
> >> message.  The latter occurs because there is a get semaphore for each
> >> channel that the callback is supposed to give.
> >>
> >> My thought is that if the callback doesn't come the semaphore should be
> >> released in the same code that sets the "get completion timeout" message.
> >
> > [...]
> >
> > I find it very hard to judge what the correct behaviour is, here.
> >
> > Maybe Jeff can comment?
> >
> >> Or is there something the operator should be able to do, short of
> >> restarting the sequencer?
> >
> > There is not, currently, but I could add something. Maybe letting the
> > programmer explicitly "recover" from timeouts is better than doing it
> > automatically, at least as long as (and assuming that) canceling pvGet
> > requests is not supported by the CA client API.
>
> I think I've run into Carl's problem before too.
>
> Could you supply the CA callback with a sequence number that would
> uniquely identify the callback as being for a particular pvGet?  This
> way, you could ignore a callback if it arrives for a pvGet that has been
> abandoned.

Right, that's a possibility.

Will look into this and see if I can get something hacked up.

One thing I hate about this is that I happen to know CA already does exactly
that, on the protocol level. It just isn't exposed in the API.

Cheers
--
Ben Franksen
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Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System