Mailbox framebuffer interface

NOTE: This particular Mailbox call is deprecated, and not guaranteed to work as expected.

Channel 1 of mailbox 0 is used.

Mailbox messages:

• Request data: The 28 most significant bits of the address of a buffer
• Response data: 0x0000000 (0x00000001 including the channel identifier)

The buffer must be 16-byte aligned as only the upper 28 bits of the address can be passed via the mailbox.

The buffer contains the following structure:

• u32: Requested width of the physical display
• u32: Requested height of the physical display
• u32: Requested width of the virtual framebuffer
• u32: Requested height of the virtual framebuffer
• u32: Pitch
• Request: Set to zero
• Response: Number of bytes between each row of the frame buffer
• u32: Requested depth (bits per pixel)
• u32: Requested X offset of the virtual framebuffer
• u32: Requested Y offset of the virtual framebuffer
• u32: Framebuffer address
• Request: Set to zero
• Response: Address of buffer allocated by VC, or zero if request fails
• u32: Framebuffer size
• Request: Set to zero
• Response: Size of buffer allocated by VC

All u32 values are little endian.