Setting options on matrices obtained from a DM

[Jed Brown]

If I get a matrix from a DM, I usually do something like

  PetscOptionsGetString(NULL,"-q1_mat_type",mtype,sizeof(mtype),NULL);
  DMGetMatrix(dm,mtype,&Jp);

so that the type can be changed on the command line (I frequently switch
between preconditioners that can use BAIJ and those that cannot).  But,
MatSetFromOptions never gets called.  If I add

  MatSetOptionsPrefix(Jp,"q1_");
  MatSetFromOptions(Jp);

after the above, then preallocation will be blown if I change anything
on the command line.  This includes setting block size for AIJ formats
(I think BAIJ should implement MatSetBlockSize and just confirm that it
matches, but that's a separate issue) so MatSetValuesBlocked cannot
even assemble the correct matrix.

To rectify this, I think that DMGetMatrix either must have enough
information to call MatSetFromOptions (like the prefix) or preallocation
should be separated from setting the sizes (so the user can get the
matrix, call MatSetFromOptions, and then ask the DM to preallocate).
In the former case, we have functionality that is essentially
inaccessible from code (PetscOptionsSetValue is a workaround).  The
latter significantly complicates matters.

Note that since the user may get multiple matrices from a DM, to be used
for different purposes, it's not okay to use the DM's prefix as the
Mat's prefix.

Any preferences here?  I'm happy with any solution in which the user
doesn't have to do an inordinate amount of work to make the following
work.

  -q1_mat_type baij
  -q1_mat_type aij -mat_no_inode
  -q1_mat_type crl


Jed

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