docs: Update the CudaPackage (build system) description (#20742)

Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

lib/spack/docs/build_systems/cudapackage.rst

@@ -9,35 +9,120 @@
 CudaPackage
 -----------
 
-Different from other packages, ``CudaPackage`` does not represent a build
+Different from other packages, ``CudaPackage`` does not represent a build system.
-system. Instead its goal is to simplify and unify usage of ``CUDA`` in other
+Instead its goal is to simplify and unify usage of ``CUDA`` in other packages by providing a ` mixin-class <https://en.wikipedia.org/wiki/Mixin>`__.
-packages.
 
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+You can find source for the package at
-Provided variants and dependencies
+`<https://github.com/spack/spack/blob/develop/lib/spack/spack/build_systems/cuda.py>`__.
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-``CudaPackage`` provides ``cuda`` variant (default to ``off``) to enable/disable
+^^^^^^^^
-``CUDA``, and ``cuda_arch`` variant to optionally specify the architecture.
+Variants
-It also declares dependencies on the ``CUDA`` package ``depends_on('cuda@...')``
+^^^^^^^^
-based on the architecture as well as specifies conflicts for certain compiler versions.
+
+This package provides the following variants:
+
+* **cuda**
+
+  This variant is used to enable/disable building with ``CUDA``. The default
+  is disabled (or ``False``).
+
+* **cuda_arch**
+
+  This variant supports the optional specification of the architecture.
+  Valid values are maintained in the ``cuda_arch_values`` property and
+  are the numeric character equivalent of the compute capability version
+  (e.g., '10' for version 1.0). Each provided value affects associated
+  ``CUDA`` dependencies and compiler conflicts.
+
+  GPUs and their compute capability versions are listed at
+  https://developer.nvidia.com/cuda-gpus .
+
+^^^^^^^^^
+Conflicts
+^^^^^^^^^
+
+Conflicts are used to prevent builds with known bugs or issues. While
+base ``CUDA`` conflicts have been included with this package, you may
+want to add more for your software.
+
+For example, if your package requires ``cuda_arch`` to be specified when
+``cuda`` is enabled, you can add the following conflict to your package
+to terminate such build attempts with a suitable message:
+
+.. code-block:: python
+
+    conflicts('cuda_arch=none', when='+cuda',
+              msg='CUDA architecture is required')
+
+Similarly, if your software does not support all versions of the property,
+you could add ``conflicts`` to your package for those versions.  For example,
+suppose your software does not work with CUDA compute capability versions
+prior to SM 5.0 (``50``).  You can add the following code to display a
+custom message should a user attempt such a build:
+
+.. code-block:: python
+
+    unsupported_cuda_archs = [
+        '10', '11', '12', '13',
+        '20', '21',
+        '30', '32', '35', '37'
+    ]
+    for value in unsupported_cuda_archs:
+        conflicts('cuda_arch={0}'.format(value), when='+cuda',
+                  msg='CUDA architecture {0} is not supported'.format(value))
+
+^^^^^^^
+Methods
+^^^^^^^
+
+This package provides one custom helper method, which is used to build
+standard CUDA compiler flags.
+
+**cuda_flags**
+
+    This built-in static method returns a list of command line flags 
+    for the chosen ``cuda_arch`` value(s).  The flags are intended to
+    be passed to the CUDA compiler driver (i.e., ``nvcc``).
+
+    This method must be explicitly called when you are creating the
+    arguments for your build in order to use the values.
 
 ^^^^^
 Usage
 ^^^^^
 
-In order to use it, just add another base class to your package, for example:
+This helper package can be added to your package by adding it as a base
+class of your package.  For example, you can add it to your
+:ref:`CMakePackage <cmakepackage>`-based package as follows:
 
 .. code-block:: python
+   :emphasize-lines: 1,7-16
 
-    class MyPackage(CMakePackage, CudaPackage):
+    class MyCudaPackage(CMakePackage, CudaPackage):
         ...
         def cmake_args(self):
             spec = self.spec
+            args = []
+            ...
             if '+cuda' in spec:
-                options.append('-DWITH_CUDA=ON')
+                # Set up the cuda macros needed by the build
-                cuda_arch = spec.variants['cuda_arch'].value
+                args.append('-DWITH_CUDA=ON')
+                cuda_arch_list = spec.variants['cuda_arch'].value
+                cuda_arch = cuda_arch_list[0]
                 if cuda_arch != 'none':
-                    options.append('-DCUDA_FLAGS=-arch=sm_{0}'.format(cuda_arch[0]))
+                    args.append('-DCUDA_FLAGS=-arch=sm_{0}'.format(cuda_arch))
             else:
-                options.append('-DWITH_CUDA=OFF')
+                # Ensure build with cuda is disabled
+                args.append('-DWITH_CUDA=OFF')
+            ...
+            return args
+
+assuming only the ``WITH_CUDA`` and ``CUDA_FLAGS`` flags are required.
+You will need to customize options as needed for your build.
+
+This example also illustrates how to check for the ``cuda`` variant using
+``self.spec`` and how to retrieve the ``cuda_arch`` variant's value, which
+is a list, using ``self.spec.variants['cuda_arch'].value``.
+
+With over 70 packages using ``CudaPackage`` as of January 2021 there are
+lots of examples to choose from to get more ideas for using this package.