Update documentation and throw and error for unsupported backends

doc/pages/user-guide/case-file.md

@@ -517,6 +517,7 @@ The following keywords are used, with the corresponding options.
   - `jacobi`, a Jacobi preconditioner. Typically used for velocity.
   - `hsmg`, a hybrid-Schwarz multigrid preconditioner. Typically used for
     pressure.
+  - `phmg`, a hybrid ph multigrid preconditioner. Typically used for pressure.
   - `ident`, an identity matrix (no preconditioner).
 * `absolute_tolerance`, tolerance criterion for convergence.
 * `max_iterations`, maximum number of iterations before giving up.

src/krylov/pc_hsmg.f90

@@ -254,8 +254,14 @@ subroutine hsmg_init(this, msh, Xh, coef, dof, gs_h, bclst, crs_pctype)
     ! Create a backend specific krylov solver
     if (present(crs_pctype)) then
        if (trim(crs_pctype) .eq. 'tamg') then
+          if (NEKO_BCKND_DEVICE .eq. 1) then
+             call neko_error('Tree-amg only supported for CPU')
+          end if
+
           allocate(this%amg_solver)
-          call this%amg_solver%init(this%ax, this%grids(1)%e%Xh, this%grids(1)%coef, this%msh, this%grids(1)%gs_h, 4, &
+
+          call this%amg_solver%init(this%ax, this%grids(1)%e%Xh, &
+               this%grids(1)%coef, this%msh, this%grids(1)%gs_h, 4, &
                this%grids(1)%bclst, 1)
        else
           call krylov_solver_factory(this%crs_solver, &            

src/krylov/precon_fctry.f90

@@ -73,6 +73,9 @@ module subroutine precon_factory(pc, type_name)
     else if (type_name(1:4) .eq. 'hsmg') then
        allocate(hsmg_t::pc)
     else if (type_name(1:4) .eq. 'phmg') then
+       if (NEKO_BCKND_DEVICE .eq. 1) then
+          call neko_error('Hybrid ph multigrid only supported for CPU')
+       end if
        allocate(phmg_t::pc)
     else if(trim(type_name) .eq. 'ident') then
        if (NEKO_BCKND_DEVICE .eq. 1) then