External ROCm: add example configuration (spack#39602)

* add an example of an external rocm configuration
* include more info
* generalize section to all GPU support

lib/spack/docs/gpu_configuration.rst

@@ -0,0 +1,113 @@
+.. Copyright 2013-2023 Lawrence Livermore National Security, LLC and other
+   Spack Project Developers. See the top-level COPYRIGHT file for details.
+
+   SPDX-License-Identifier: (Apache-2.0 OR MIT)
+
+==========================
+Using External GPU Support
+==========================
+
+Many packages come with a ``+cuda`` or ``+rocm`` variant. With no added
+configuration Spack will download and install the needed components.
+It may be preferable to use existing system support: the following sections
+help with using a system installation of GPU libraries.
+
+-----------------------------------
+Using an External ROCm Installation
+-----------------------------------
+
+Spack breaks down ROCm into many separate component packages. The following
+is an example ``packages.yaml`` that organizes a consistent set of ROCm
+components for use by dependent packages:
+
+.. code-block:: yaml
+
+   packages:
+     all:
+       compiler: [rocmcc@=5.3.0]
+       variants: amdgpu_target=gfx90a
+     hip:
+       buildable: false
+       externals:
+       - spec: hip@5.3.0
+         prefix: /opt/rocm-5.3.0/hip
+     hsa-rocr-dev:
+       buildable: false
+       externals:
+       - spec: hsa-rocr-dev@5.3.0
+         prefix: /opt/rocm-5.3.0/
+     llvm-amdgpu:
+       buildable: false
+       externals:
+       - spec: llvm-amdgpu@5.3.0
+         prefix: /opt/rocm-5.3.0/llvm/
+     comgr:
+       buildable: false
+       externals:
+       - spec: comgr@5.3.0
+         prefix: /opt/rocm-5.3.0/
+     hipsparse:
+       buildable: false
+       externals:
+       - spec: hipsparse@5.3.0
+         prefix: /opt/rocm-5.3.0/
+     hipblas:
+       buildable: false
+       externals:
+       - spec: hipblas@5.3.0
+         prefix: /opt/rocm-5.3.0/
+     rocblas:
+       buildable: false
+       externals:
+       - spec: rocblas@5.3.0
+         prefix: /opt/rocm-5.3.0/
+     rocprim:
+       buildable: false
+       externals:
+       - spec: rocprim@5.3.0
+         prefix: /opt/rocm-5.3.0/rocprim/
+
+This is in combination with the following compiler definition:
+
+.. code-block:: yaml
+
+   compilers:
+   - compiler:
+       spec: rocmcc@=5.3.0
+       paths:
+         cc: /opt/rocm-5.3.0/bin/amdclang
+         cxx: /opt/rocm-5.3.0/bin/amdclang++
+         f77: null
+         fc: /opt/rocm-5.3.0/bin/amdflang
+       operating_system: rhel8
+       target: x86_64
+
+This includes the following considerations:
+
+- Each of the listed externals specifies ``buildable: false`` to force Spack
+  to use only the externals we defined.
+- ``spack external find`` can automatically locate some of the ``hip``/``rocm``
+  packages, but not all of them, and furthermore not in a manner that
+  guarantees a complementary set if multiple ROCm installations are available.
+- The ``prefix`` is the same for several components, but note that others
+  require listing one of the subdirectories as a prefix.
+
+-----------------------------------
+Using an External CUDA Installation
+-----------------------------------
+
+CUDA is split into fewer components and is simpler to specify:
+
+.. code-block:: yaml
+
+   packages:
+     all:
+       variants:
+       - cuda_arch=70
+     cuda:
+       buildable: false
+       externals:
+       - spec: cuda@11.0.2
+         prefix: /opt/cuda/cuda-11.0.2/
+
+where ``/opt/cuda/cuda-11.0.2/lib/`` contains ``libcudart.so``.

lib/spack/docs/index.rst

@@ -77,6 +77,7 @@ or refer to the full manual below.
    extensions
    pipelines
    signing
+   gpu_configuration
 
 .. toctree::
    :maxdepth: 2