Specifying buffer usage at creation time

This applies to both shm buffers and dma-bufs but dma-bufs are more interesting.

When creating a buffer via linux-dmabuf, the compositor cannot know how the client intends to use the buffer. The client could want to use the buffer

• as a framebuffer and/or
• for sampling.

Therefore, when the client calls

• create or
• create_immed,

the compositor cannot actually fully import the buffer. Even if those requests signal success, using the buffers might later fail if the compositor has to create additional data structures.

---

Therefore, I believe that it would be useful to add a new request to linux-dmabuf:

  <interface name="zwp_linux_buffer_params_v1" version="5">
    <enum name="error">
      <entry name="no_usage" value="8"
             summary="tried to create a buffer without a usage"/>
    </enum>

    <enum name="usage" bitfield="true">
      <entry name="rendering" value="1" summary="the buffer will be rendered-to"/>
      <entry name="sampling" value="2" summary="the buffer will be sampled-from"/>
    </enum>

    <request name="add_usage" since="5">
      <description summary="add a usage to this buffer">
        This request specifies in which ways the created buffer will be used. It is a
        protocol error to attempt to use the buffer in a way not specified via this
        request.

        The client must request at least one usage before attempting to create the buffer.
        Otherwise, the no_usage error is raised.

        This request allows the compositor to determine at create time if the dma-buf
        can be used for the intended purpose.
      </description>
      <arg name="usage" type="uint" enum="usage" summary="see enum usage"/>
    </request>
  </interface>