Re: [PATCH 2/2] gpu: nova-core: gsp: cmdq: increase MSGQ_NUM_PAGES to 64

[Alexandre Courbot]

On Wed Nov 11, 2026 at 10:37 AM JST, Eliot Courtney wrote:
> In the msgq v2 protocol, which the r000 firmware uses, the queue
> pointers are kept in u32 registers and increase monotonically until they
> wrap at 2^32.
>
> `MSGQ_NUM_PAGES` is currently 63 and does not divide 2^32. This means
> that on msgq v2, the slot can jump backwards when the counters wrap.
> For example:
>
>     (2^32 - 2) % 63 == 2
>     (2^32 - 1) % 63 == 3
>              0 % 63 == 0    (slot 0 instead of slot 4)
>
> The writer can then overwrite slots before the reader has read them.
>
> Increase `MSGQ_NUM_PAGES` to 64 and check at compile time that
> `MSGQ_NUM_PAGES` divides 2^32.

Have we tested that this was also working on msgq v0?

>
> Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
> ---
>  drivers/gpu/nova-core/gsp/cmdq.rs | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
> index 33074bb53e8c..240ab42db557 100644
> --- a/drivers/gpu/nova-core/gsp/cmdq.rs
> +++ b/drivers/gpu/nova-core/gsp/cmdq.rs
> @@ -84,7 +84,15 @@ trait CommandElement {
>  }
>  
>  /// Number of GSP pages making the [`Msgq`].
> -pub(crate) const MSGQ_NUM_PAGES: u32 = 0x3f;
> +pub(crate) const MSGQ_NUM_PAGES: u32 = 64;
> +
> +// From version 2 of the msgq protocol, the queue pointers increase
> +// monotonically in `u32` counters until they wrap. If `MSGQ_NUM_PAGES` does not
> +// divide `1 << 32`, then the slot, `counter % MSGQ_NUM_PAGES`, can jump
> +// backwards when wrapping, which could overwrite slots that still need to be
> +// read. So, require that the last counter value, `u32::MAX`, maps to the last
> +// slot.

That's a pretty convoluted way to say the number of slots must be a
power of 2 representable as a u32. :)

> +static_assert!(u32::MAX % MSGQ_NUM_PAGES == MSGQ_NUM_PAGES - 1);

Same here, can't we use `MSGQ_NUM_PAGES.is_power_of_two()`?