Fix hardcoded alignment value and zero-init padding

Author: andyleiserson

wgpu-core/src/device/resource.rs

@@ -173,6 +173,9 @@ pub struct ExternalTextureParams {
 
 const _: () =
     assert!(size_of::<ExternalTextureParams>() == wgpu_hal::EXTERNAL_TEXTURE_PARAMS_SIZE as usize);
+const EXTERNAL_TEXTURE_PARAMS_BUFFER_SIZE: usize = {
+    size_of::<ExternalTextureParams>().next_multiple_of(wgpu_hal::UNIVERSAL_BUFFER_SIZE_ALIGNMENT as usize)
+};
 
 impl ExternalTextureParams {
     pub fn from_desc<L>(desc: &wgt::ExternalTextureDescriptor<L>) -> Self {
@@ -587,7 +590,7 @@ impl Device {
                         Some("(wgpu internal) default external texture params buffer"),
                         instance_flags,
                     ),
-                    size: align_to(size_of::<ExternalTextureParams>() as _, 256),
+                    size: EXTERNAL_TEXTURE_PARAMS_BUFFER_SIZE as _,
                     usage: wgt::BufferUses::COPY_DST | wgt::BufferUses::UNIFORM,
                     memory_flags: hal::MemoryFlags::empty(),
                 })
@@ -718,7 +721,8 @@ impl Device {
             _padding: Default::default(),
         };
         let mut staging_buffer =
-            StagingBuffer::new(self, wgt::BufferSize::new(size_of_val(&data) as _).unwrap())?;
+            StagingBuffer::new(self, wgt::BufferSize::new(EXTERNAL_TEXTURE_PARAMS_BUFFER_SIZE as _).unwrap())?;
+        staging_buffer.write_zeros();
         staging_buffer.write(bytemuck::bytes_of(&data));
         let staging_buffer = staging_buffer.flush();
 

wgpu-hal/src/lib.rs

@@ -330,7 +330,13 @@ pub const MAX_MIP_LEVELS: u32 = 16;
 /// Size of a single occlusion/timestamp query, when copied into a buffer, in bytes.
 /// cbindgen:ignore
 pub const QUERY_SIZE: wgt::BufferAddress = 8;
+// The struct itself is defined in core, but we need to know the size.
+// There is a const assert for correctness located with the struct definition.
 pub const EXTERNAL_TEXTURE_PARAMS_SIZE: wgt::BufferAddress = 208;
+/// Universally safe value for buffer size alignment.
+///
+/// This is determined by `D3D12_CONSTANT_BUFFER_DATA_PLACEMENT_ALIGNMENT`.
+pub const UNIVERSAL_BUFFER_SIZE_ALIGNMENT: wgt::BufferAddress = 256;
 
 pub type Label<'a> = Option<&'a str>;
 pub type MemoryRange = Range<wgt::BufferAddress>;