Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Wed, 6 Jan 2010 13:25:23 -0600

Hi Dirk,

On Wednesday 06 January 2010 03:37:53 Dirk Zimoch wrote:
> It is a pity that the Bx fields of mbboDirect do not update when VAL is
> written. It seems mbboDirect has not been designed to have VAL written.
> But then neither DOL not the record_init() code makes much sense. Maybe
> this is an issue for the next Codeathon. Provided we have defined what
> the "expected behaviour" is.

To add a little history to the discussion, the mbb*Direct record types were 
created from the mbb* record types at JLAB.  I suspect that their deficiencies 
have not been corrected because their original author lost interest and nobody 
else has been sufficiently motivated to fix them before now.

I have a couple of private emails about a patch to the mbboDirect record from 
September 2008, but the suggestion at the time was that more changes were 
still needed, which was one of the reasons why I didn't apply the change.

I would certainly encourage anyone who wants to work on improvements to do so 
(and publicize that you are doing so), although if you make significant 
changes to the configuration fields or runtime behaviour it might be advisable 
to rename the record type at the same time so that existing IOCs which may 
rely on the current behaviour do not break when upgraded to a release 
containing the new version.  It is then trivial to deprecate the old record 
types and unbundle them from Base while still making them available for 
existing installations to use.

> I propose the following behavior:

I haven't examined your specific suggestions, I'm happy to let you, Stephanie 
and others to work on the details.  I would like reference documentation to 
accompany any new or modified record type.

> What about extending the mbb* records to 32 bits?

I don't understand what you mean by that; the VAL field of the mbb[io] records 
is a DBF_ENUM (i.e. an index into a list of choices) and you can't change the 
underlying type for the index value.  The xxVL, RVAL and MASK fields are 
DBF_ULONG and thus 32 bits wide already.

If you meant to increase the number of choices possible from 16 to 32, I'm 
afraid that is currently limited by the CA network protocol and API which was 
only designed to transport at most 16 state strings.  That limitation is 
likely to be removed at some point, but it will not be possible to make the 
result backwards compatible so existing CA clients will have to be modified to 
use more than 16 states.

If you meant extend the mbb[io]Direct records to 32 bits though, I think that 
has already been done by at least one site and does seem worthwhile.

> Heretic question: What about combining the features of mbb[io]Direct
> with mbb[io], i.e. adding Bx fields to mbb[io] records and get rid of
> mbb[io]Direct?

I don't see any major advantage to doing this, since the record types are 
really very different in purpose.  The mbb[io] records allow a series of state 
names to be associated with specific bit patterns, whereas the mbb[io]Direct 
records allow individual bits in a word to be separated out and combined.  The 
only overlap between these is in the I/O of the raw value; sharing device 
support between record types will need a whole new record/device interface to 
be developed though...

- Andrew
-- 
The best FOSS code is written to be read by other humans -- Harald Welte

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System