Re: [PATCH v5 2/9] gpu: nova-core: gsp: add mechanism to wait for space on command queue

["Alexandre Courbot" <acourbot@nvidia.com>]

On Thu Mar 5, 2026 at 8:16 PM JST, Danilo Krummrich wrote:
> @Alex: It also seems that this is based on broken code, i.e. I noticed how the
> DMA read is done in this case in e.g. gsp_read_ptr().
>
> 	fn cpu_read_ptr(&self) -> u32 {
> 	    let gsp_mem = self.0.start_ptr();
> 	
> 	    // SAFETY:
> 	    //  - The ['CoherentAllocation'] contains at least one object.
> 	    //  - By the invariants of CoherentAllocation the pointer is valid.
> 	    (unsafe { (*gsp_mem).cpuq.rx.read_ptr() } % MSGQ_NUM_PAGES)
> 	}
>
> Why isn't this using dma_read!()? I think creating this reference is UB.

We can't - technically we would have to have the `dma_read` in `cmdq.rs`
so it can access the `CoherentAllocation` (and do an unwrap in the
process):

    dma_read!(self.0, 0, .gspq.rx.0.readPtr).unwrap()

... but that cannot be done as `MsgqRxHeader` is part of the bindings
(i.e. in `fw.rs`) and thus its internal fields are not visible to
`cmdq.rs`, as per our policy of making the bindigns opaque.

This can probably be done better with I/O projections, but for now we
have to do the read_volatile by ourselves. What makes this reference UB
btw?