Fix loading on unsupported platforms (#459)

* Gate getpagesize call behind function

* Make `functional` require a fully-supported GPU

lib/mtl/buffer.jl

@@ -49,16 +49,23 @@ function MTLBuffer(dev::MTLDevice, bytesize::Integer, ptr::Ptr;
     return MTLBuffer(ptr)
 end
 
-const PAGESIZE = ccall(:getpagesize, Cint, ())
+const _page_size::Ref{Int} = Ref{Int}(0)
+function page_size()
+    if _page_size[] == 0
+        _page_size[] = Int(ccall(:getpagesize, Cint, ()))
+    end
+    _page_size[]
+end
+
 function can_alloc_nocopy(ptr::Ptr, bytesize::Integer)
     # newBufferWithBytesNoCopy has several restrictions:
     ## the pointer has to be page-aligned
-    if Int64(ptr) % PAGESIZE != 0
+    if Int(ptr) % page_size() != 0
         return false
     end
     ## the new buffer needs to be page-aligned
     ## XXX: on macOS 14, this doesn't seem required; is this a documentation issue?
-    if bytesize % PAGESIZE != 0
+    if bytesize % page_size() != 0
         return false
     end
     return true

src/initialization.jl

@@ -1,5 +1,16 @@
-const _functional = Ref{Bool}(false)
-functional() = _functional[]
+# Starts at `nothing`. Only false when it's determined
+const _functional = Ref{Union{Nothing,Bool}}(false)
+
+function functional()
+    if isnothing(_functional[])
+        dev = device()
+
+        _functional[] =
+            supports_family(dev, MTL.MTLGPUFamilyApple7) &&
+            supports_family(dev, MTL.MTLGPUFamilyMetal3)
+    end
+    _functional[]
+end
 
 function __init__()
     precompiling = ccall(:jl_generating_output, Cint, ()) != 0
@@ -39,7 +50,10 @@ function __init__()
         load_framework("CoreGraphics")
         ver = MTL.MTLCompileOptions().languageVersion
         @debug "Successfully loaded Metal; targeting v$ver."
-        _functional[] = true
+
+        # Successful loading of CoreGraphics means there's a
+        # chance the graphics device is supported
+        _functional[] = nothing
     catch err
         @error "Failed to load Metal" exception=(err,catch_backtrace())
         return