gpu: nova-core: gsp: fix improper handling of empty slot in cmdq

The current code hands out buffers that go all the way up to and
including `rx - 1`, but we need to maintain an empty slot to prevent the
ring buffer from wrapping around into having 'tx == rx', which means
empty.

Also add more rigorous no-panic proofs.

Fixes: 75f6b1de8133 ("gpu: nova-core: gsp: Add GSP command queue bindings and handling")
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260129-nova-core-cmdq1-v3-4-2ede85493a27@nvidia.com
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

1 files changed, 20 insertions, 14 deletions

diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
index b88ff8ebc098..333bf0125d74 100644
--- a/drivers/gpu/nova-core/gsp/cmdq.rs
+++ b/drivers/gpu/nova-core/gsp/cmdq.rs
@@ -227,21 +227,27 @@ impl DmaGspMem {
         // 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 {
-            // The area from `tx` up to the end of the ring, and from the beginning of the ring up
-            // to `rx`, minus one unit, belongs to the driver.
-            if rx == 0 {
-                let last = after_tx.len() - 1;
-                (&mut after_tx[..last], &mut [])
-            } else {
-                (after_tx, &mut before_tx[..rx])
-            }
+        // The area starting at `tx` and ending at `rx - 2` modulo MSGQ_NUM_PAGES, inclusive,
+        // belongs to the driver for writing.
+
+        if rx == 0 {
+            // Since `rx` is zero, leave an empty slot at end of the buffer.
+            let last = after_tx.len() - 1;
+            (&mut after_tx[..last], &mut [])
+        } else if rx <= tx {
+            // The area is discontiguous and we leave an empty slot before `rx`.
+            // PANIC:
+            // - The index `rx - 1` is non-negative because `rx != 0` in this branch.
+            // - The index does not exceed `before_tx.len()` (which equals `tx`) because
+            //   `rx <= tx` in this branch.
+            (after_tx, &mut before_tx[..(rx - 1)])
         } else {
-            // The area from `tx` to `rx`, minus one unit, belongs to the driver.
-            //
-            // PANIC: per the invariants of `cpu_write_ptr` and `gsp_read_ptr`, `rx` and `tx` are
-            // `<= MSGQ_NUM_PAGES`, and the test above ensured that `rx > tx`.
-            (after_tx.split_at_mut(rx - tx).0, &mut [])
+            // The area is contiguous and we leave an empty slot before `rx`.
+            // PANIC:
+            // - The index `rx - tx - 1` is non-negative because `rx > tx` in this branch.
+            // - The index does not exceed `after_tx.len()` (which is `MSGQ_NUM_PAGES - tx`)
+            //   because `rx < MSGQ_NUM_PAGES` by the `gsp_read_ptr` invariant.
+            (&mut after_tx[..(rx - tx - 1)], &mut [])
         }
     }