How to retrieve physical address of struct instance

[daft-panda]

I would like to communicate with the VideoCore using the mailbox, and can so pre-MMU. However, starting from the last tutorial, I cannot figure out how to get a pointer accessible by the VideoCore mailbox. I did find the DMA example within the previous version of the tutorials, based on which I managed to define a non-cacheable memory region (master...daft-panda:rust-raspberrypi-OS-tutorials:master#diff-24c0e4c84c820d58e9a59613c2e7bef643da6b79a3db7dacedcfc6d3b5358b3dL37), memory map it with the MMU and create an dedicated rust allocator for that region.

However getting a physical memory address pointer eludes me. Failing this I tried to identity map the DMA region by rummaging in the Ruby code, however upon boot the isValid flag becomes false in the stage 1 page descriptor for unknown reasons.