Matrices in MTLVertexDescriptor

I'm trying to write vertex descriptor for my vertex shader which takes following struct as stage_in input.

struct VertexIn {
      float2x2 foo;
}

vertex float4 vertexShader(const VertexIn in [[stage_in]]) {...}

Now when defining vertex descriptor's attribute what should be the MTLVertexFormat? vertexDescriptor.attributes[0].format = ???

I went through the documentation, I didn't find any enum case for Matrices. Is it fine if say set format as float2 and give size 2 * size of float2?