[petsc-dev] On remaining libMesh-PETsc fieldsplit interface cleanup

[Smith, Barry F.]

> On Apr 1, 2019, at 8:19 PM, Matthew Knepley via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> 
> On Mon, Apr 1, 2019 at 9:12 PM Boris Boutkov via petsc-dev <petsc-dev at mcs.anl.gov> wrote:
> Hello all,
> 
> I've been working on the libMesh - PETSc interface for utilizing gmg/fieldsplit on the command line by creating DMShells; things are seemingly in pretty good shape at this point, so thanks much for the support along the way!
> 
> The last lingering issue of the implementation is that I still have a little bit of dependency on PETSc private/dmimpl.h. In short, during our implementation of DMCreateSubDM I try and get some of the parent DMs function pointers, specifically dm->ops->{coarsen, refine, createinterpolation, createrestriction, and createsubdm}, which then I set for the subDM using DMShellSet* by passing in the parent DM pointer directly.
> 
> I'd like to remove this private dmimpl.h dependency but removing the header gives me compilation errors - "invalid use of incomplete type" pointing to PETsc DM struct for the above calls. As far as I understand it, this means that I would need public API similar to the DMShellSet* methods but now instead DMShellGet*.
> 
> If the above's the case, any reason this doesn't exist yet outside of lack of prior need? And how much work would introducing this require? Or of course please let me know if there's some simple way around this that I may be missing.
> 
> No, we just needed someone to ask. We will do this exactly like we do MATSHELL. We have to create an Enum that indexes
> the DM methods. Then you can pass the Enum value when you Set/Get the methods. Its not hard, just some coding. Want to try :)

   Actually this is not the way DMShell works, DMShell works the way PCSHELL works, each set function is its own function (for example DMShellSetCreateGlobalVector)  and there is no enum of operations (like MatOperations). Given the current DMSHELL API it makes sense to just provide DMShellGetXX() for each DM operation.

   Barry

  Well you may well ask, why we have two different approaches for different PETSc objects/classes. I am asking. And well you may.

  I don't have any rational for having two different approaches, lack of good code reviews in the past? Should we evolve towards using the same approach for all XXXSHELL subclasses in the future? Perhaps. 


   I 
> 
>    Thanks,
> 
>      Matt
> Thanks as always for your time,
> 
> - Boris
> 
> 
> 
> 
> 
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener
> 
> https://www.cse.buffalo.edu/~knepley/