vulkan: do a better job finding the vulkan xml

Hard-code some common alternative locations to the vulkan XML, and
improve the error message otherwise.

Fixes https://github.com/haasn/libplacebo/issues/75
Fixes #95

README.md

@@ -210,6 +210,14 @@ A full list of optional dependencies each feature requires:
 - **shaderc**: `libshaderc`
 - **vulkan**: `libvulkan`, `python3-mako`
 
+#### Vulkan support
+
+Because the vulkan backend requires on code generation at compile time,
+`python3-mako` is a hard dependency of the build system. In addition to this,
+the path to the Vulkan registry (`vk.xml`) must be locatable, ideally by
+explicitly providing it via the `-Dvulkan-registry=/path/to/vk.xml` option,
+unless it can be found in one of the built-in hard-coded locations.
+
 ### Configuring
 
 To get a list of configuration options supported by libplacebo, after running

meson_options.txt

@@ -5,8 +5,7 @@ option('vulkan', type: 'feature', value: 'auto',
 option('vulkan-link', type: 'boolean', value: true,
        description: 'Link directly againt vkGetInstanceProcAddr from libvulkan.so')
 
-option('vulkan-registry', type: 'string',
-       value: '/usr/share/vulkan/registry/vk.xml',
+option('vulkan-registry', type: 'string', value: '',
        description: 'Path to vulkan XML registry (for code generation)')
 
 option('opengl', type: 'feature', value: 'auto',

src/vulkan/utils_gen.py

 #!/usr/bin/env python3
+#
 # This file is part of libplacebo.
 #
 # libplacebo is free software; you can redistribute it and/or
@@ -14,6 +15,7 @@
 # You should have received a copy of the GNU Lesser General Public
 # License along with libplacebo.  If not, see <http://www.gnu.org/licenses/>.
 
+import os.path
 import sys
 import xml.etree.ElementTree as ET
 from mako.template import Template
@@ -86,11 +88,32 @@ def get_vkstructs(registry):
             yield Obj(stype = stype.attrib['values'],
                       name = e.attrib['name'])
 
+def find_registry_xml():
+    registry_paths = [
+        '/usr/share/vulkan/registry/vk.xml',
+        '%VULKAN_SDK%/share/vulkan/registry/vk.xml',
+        '$MINGW_PREFIX/share/vulkan/registry/vk.xml',
+    ]
+
+    for p in registry_paths:
+        path = os.path.expandvars(p)
+        if os.path.isfile(path):
+            print('Found vk.xml: {0}'.format(path))
+            return path
+
+    print('Could not find the vulkan registry (vk.xml), please specify its '
+          'location manually using the -Dvulkan-registry=/path/to/vk.xml '
+          'option!', file=sys.stderr)
+    sys.exit(1)
+
 if __name__ == '__main__':
     assert len(sys.argv) == 3
     xmlfile = sys.argv[1]
     outfile = sys.argv[2]
 
+    if not xmlfile or xmfile == '':
+        xmlfile = find_registry_xml()
+
     registry = ET.parse(xmlfile)
     with open(outfile, 'w') as f:
         f.write(TEMPLATE.render(