linux-kernel - Re: [PATCH v2 2/4] gpu: nova-core: gsp: clarify comments about invariants and pointer roles

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Message-Id: <DFYPX3TNI3Y2.38MQUIWHHR9Z1@garyguo.net>
Date: Mon, 26 Jan 2026 18:04:47 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Eliot Courtney" <ecourtney@...dia.com>, "Danilo Krummrich"
 <dakr@...nel.org>, "Alexandre Courbot" <acourbot@...dia.com>, "Alice Ryhl"
 <aliceryhl@...gle.com>, "David Airlie" <airlied@...il.com>, "Simona Vetter"
 <simona@...ll.ch>, "Alistair Popple" <apopple@...dia.com>
Cc: <nouveau@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>,
 <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/4] gpu: nova-core: gsp: clarify comments about
 invariants and pointer roles

On Fri Jan 23, 2026 at 12:12 PM GMT, Eliot Courtney wrote:
> Disambiguate a few things in comments in cmdq.rs.
>
> Signed-off-by: Eliot Courtney <ecourtney@...dia.com>
> ---
>  drivers/gpu/nova-core/gsp/cmdq.rs | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
> index f139aad7af3f..09c28eeb6f12 100644
> --- a/drivers/gpu/nova-core/gsp/cmdq.rs
> +++ b/drivers/gpu/nova-core/gsp/cmdq.rs
> @@ -161,12 +161,14 @@ struct GspMem {
>      /// Self-mapping page table entries.
>      ptes: PteArray<{ GSP_PAGE_SIZE / size_of::<u64>() }>,
>      /// CPU queue: the driver writes commands here, and the GSP reads them. It also contains the
> -    /// write and read pointers that the CPU updates.
> +    /// write and read pointers that the CPU updates. This means that the read pointer here is an
> +    /// index into the GSP queue.
>      ///
>      /// This member is read-only for the GSP.
>      cpuq: Msgq,
>      /// GSP queue: the GSP writes messages here, and the driver reads them. It also contains the
> -    /// write and read pointers that the GSP updates.
> +    /// write and read pointers that the GSP updates. This means that the read pointer here is an
> +    /// index into the CPU queue.
>      ///
>      /// This member is read-only for the driver.
>      gspq: Msgq,
> @@ -222,7 +224,7 @@ fn new(dev: &device::Device<device::Bound>) -> Result<Self> {
>          // - We will only access the driver-owned part of the shared memory.
>          // - Per the safety statement of the function, no concurrent access will be performed.
>          let gsp_mem = &mut unsafe { self.0.as_slice_mut(0, 1) }.unwrap()[0];
> -        // PANIC: per the invariant of `cpu_write_ptr`, `tx` is `<= MSGQ_NUM_PAGES`.
> +        // PANIC: per the invariant of `cpu_write_ptr`, `tx` is `< MSGQ_NUM_PAGES`.

Can this just be `tx < MSGQ_NUM_PAGES`?

>          let (before_tx, after_tx) = gsp_mem.cpuq.msgq.data.split_at_mut(tx);
>  
>          if rx <= tx {
> @@ -257,7 +259,7 @@ fn new(dev: &device::Device<device::Bound>) -> Result<Self> {
>          // - We will only access the driver-owned part of the shared memory.
>          // - Per the safety statement of the function, no concurrent access will be performed.
>          let gsp_mem = &unsafe { self.0.as_slice(0, 1) }.unwrap()[0];
> -        // PANIC: per the invariant of `cpu_read_ptr`, `xx` is `<= MSGQ_NUM_PAGES`.
> +        // PANIC: per the invariant of `cpu_read_ptr`, `rx` is `< MSGQ_NUM_PAGES`.
>          let (before_rx, after_rx) = gsp_mem.gspq.msgq.data.split_at(rx);
>  
>          match tx.cmp(&rx) {
> @@ -315,7 +317,7 @@ fn allocate_command(&mut self, size: usize) -> Result<GspCommand<'_>> {
>      //
>      // # Invariants
>      //
> -    // - The returned value is between `0` and `MSGQ_NUM_PAGES`.
> +    // - The returned value is between `0` and `MSGQ_NUM_PAGES - 1`, inclusive.

I wonder if this can be `is within 0..MSGQ_NUM_PAGES`. What do others think?

Best,
Gary

>      fn gsp_write_ptr(&self) -> u32 {
>          let gsp_mem = self.0.start_ptr();
>  
> @@ -329,7 +331,7 @@ fn gsp_write_ptr(&self) -> u32 {
>      //
>      // # Invariants
>      //
> -    // - The returned value is between `0` and `MSGQ_NUM_PAGES`.
> +    // - The returned value is between `0` and `MSGQ_NUM_PAGES - 1`, inclusive.
>      fn gsp_read_ptr(&self) -> u32 {
>          let gsp_mem = self.0.start_ptr();
>  
> @@ -343,7 +345,7 @@ fn gsp_read_ptr(&self) -> u32 {
>      //
>      // # Invariants
>      //
> -    // - The returned value is between `0` and `MSGQ_NUM_PAGES`.
> +    // - The returned value is between `0` and `MSGQ_NUM_PAGES - 1`, inclusive.
>      fn cpu_read_ptr(&self) -> u32 {
>          let gsp_mem = self.0.start_ptr();
>  
> @@ -372,7 +374,7 @@ fn advance_cpu_read_ptr(&mut self, elem_count: u32) {
>      //
>      // # Invariants
>      //
> -    // - The returned value is between `0` and `MSGQ_NUM_PAGES`.
> +    // - The returned value is between `0` and `MSGQ_NUM_PAGES - 1`, inclusive.
>      fn cpu_write_ptr(&self) -> u32 {
>          let gsp_mem = self.0.start_ptr();
>