Re: [PATCH 03/13] gpu: nova-core: fsp: try to enforce exclusive access to FSP channel

[Gary Guo]

On Mon Jun 15, 2026 at 3:40 PM BST, Eliot Courtney wrote:
> Currently, `send_msg` assumes that the channel to FSP is free to write
> into. But, it might not be. Both the kernel driver and GSP communicate
> with FSP. The way they should attempt to keep exclusive access to this
> channel to FSP is by making sure they don't try to start writing if
> there's pending data until the full round trip has finished.
>
> Signed-off-by: Eliot Courtney <ecourtney@xxxxxxxxxx>
> ---
>  drivers/gpu/nova-core/falcon/fsp.rs | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
>
> diff --git a/drivers/gpu/nova-core/falcon/fsp.rs b/drivers/gpu/nova-core/falcon/fsp.rs
> index 21eaa8e261ce..cdb476894e1a 100644
> --- a/drivers/gpu/nova-core/falcon/fsp.rs
> +++ b/drivers/gpu/nova-core/falcon/fsp.rs
> @@ -125,6 +125,26 @@ fn poll_msgq(&self, bar: Bar0<'_>) -> Result<u32> {
>          }
>      }
>  
> +    /// Both the kernel driver and GSP talk to FSP. Try to ensure exclusive access to the FSP is
> +    /// enforced by making sure there is not a pending message already sent to FSP, and that there
> +    /// is no pending message from FSP to be read.
> +    fn wait_until_ready(&mut self, bar: Bar0<'_>) -> Result {
> +        read_poll_timeout(
> +            || {
> +                let qhead = bar.read(regs::NV_PFSP_QUEUE_HEAD::at(0)).address();
> +                let qtail = bar.read(regs::NV_PFSP_QUEUE_TAIL::at(0)).address();
> +                let mhead = bar.read(regs::NV_PFSP_MSGQ_HEAD::at(0)).val();
> +                let mtail = bar.read(regs::NV_PFSP_MSGQ_TAIL::at(0)).val();

How does this prevent race between kernel and GSP when initiating FSP
communcation?

Best,
Gary

> +
> +                Ok(qhead == qtail && mhead == mtail)
> +            },
> +            |&ready| ready,
> +            Delta::from_millis(10),
> +            Delta::from_millis(FSP_MSG_TIMEOUT_MS),
> +        )?;
> +        Ok(())
> +    }
> +
>      /// Writes `packet` to FSP EMEM and updates the queue pointers to notify FSP.
>      ///
>      /// Returns `EINVAL` if `packet` is empty or its length is not 4-byte aligned.
> @@ -133,6 +153,9 @@ pub(crate) fn send_msg(&mut self, bar: Bar0<'_>, packet: &[u8]) -> Result {
>              return Err(EINVAL);
>          }
>  
> +        // Try to make sure we have exclusive access to the FSP at this point.
> +        self.wait_until_ready(bar)?;
> +
>          self.write_emem(bar, packet)?;
>  
>          // Update queue pointers. TAIL points at the last DWORD written.