VRAM width per memory channel for LPDDR5

AMDGPU drivers report 256-bits of VRAM width on APUs using LPDDR5.
However, according to the product specifications, VRAM width is 128-bits.

• VanGogh
  • https://linux-hardware.org/?probe=1c71987bd5&log=dmesg
  • https://www.steamdeck.com/en/tech
• Rembrandt/Yellow Carp
  • https://linux-hardware.org/?probe=1c81e8c322&log=dmesg
• Mendocino (AMDGPU: LPDDR5 128-bits, actually: LPDDR5 64-bits)
  • http://linux-hardware.org/?probe=f48f292cd5&log=dmesg

I think it is because the code in the drivers/gpu/drm/amd/amdgpu/amdgpu_atomfirmware.c fixes the bus width per memory channel at 64-bits.

		if (adev->flags & AMD_IS_APU) {
			igp_info = (union igp_info *)
				(mode_info->atom_context->bios + data_offset);
			switch (frev) {
			case 1:
				switch (crev) {
				case 11:
				case 12:
					mem_channel_number = igp_info->v11.umachannelnumber;
					if (!mem_channel_number)
						mem_channel_number = 1;
					/* channel width is 64 */
					if (vram_width)
						*vram_width = mem_channel_number * 64;
					mem_type = igp_info->v11.memorytype;
					if (vram_type)
						*vram_type = convert_atom_mem_type_to_vram_type(adev, mem_type);
					break;
				default:
					return -EINVAL;
				}
				break;
			case 2:
				switch (crev) {
				case 1:
				case 2:
					mem_channel_number = igp_info->v21.umachannelnumber;
					if (!mem_channel_number)
						mem_channel_number = 1;
					/* channel width is 64 */
					if (vram_width)
						*vram_width = mem_channel_number * 64;
					mem_type = igp_info->v21.memorytype;
					if (vram_type)
						*vram_type = convert_atom_mem_type_to_vram_type(adev, mem_type);
					break;
				default:
					return -EINVAL;
				}
				break;
			default:
				return -EINVAL;
			}