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

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Message-Id: <20260122-nova-core-cmdq1-v1-2-7f8fe4683f11@nvidia.com>
Date: Thu, 22 Jan 2026 11:59:05 +0900
From: Eliot Courtney <ecourtney@...dia.com>
To: 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, 
 Eliot Courtney <ecourtney@...dia.com>
Subject: [PATCH 2/4] gpu: nova-core: gsp: clarify comments about invariants
 and pointer roles

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`.
         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.
     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();
 

-- 
2.52.0