[PATCH 4/4] gpu: nova-core: gsp: fix improper indexing in driver_read_area

[Eliot Courtney <ecourtney@nvidia.com>]

The current code indexes into `after_rx` using `tx` which is an index
for the whole buffer, not the split buffer `after_rx`.

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>
---
 drivers/gpu/nova-core/gsp/cmdq.rs | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/nova-core/gsp/cmdq.rs b/drivers/gpu/nova-core/gsp/cmdq.rs
index b6d6093e3ac0..5ca1d757d4a3 100644
--- a/drivers/gpu/nova-core/gsp/cmdq.rs
+++ b/drivers/gpu/nova-core/gsp/cmdq.rs
@@ -1,7 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 
 use core::{
-    cmp,
     mem,
     sync::atomic::{
         fence,
@@ -265,10 +264,18 @@ fn new(dev: &device::Device<device::Bound>) -> Result<Self> {
         // 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) {
-            cmp::Ordering::Equal => (&after_rx[0..0], &after_rx[0..0]),
-            cmp::Ordering::Greater => (&after_rx[..tx], &before_rx[0..0]),
-            cmp::Ordering::Less => (after_rx, &before_rx[..tx]),
+        // The area starting at `rx` and ending at `tx - 1` modulo MSGQ_NUM_PAGES, inclusive,
+        // belongs to the driver for reading.
+        if rx <= tx {
+            // The area is contiguous.
+            // PANIC: since `rx <= tx` we have `tx - rx >= 0`
+            // PANIC: since `tx < MSGQ_NUM_PAGES && after_rx.len() == MSGQ_NUM_PAGES - rx`:
+            //   `tx - rx <= MSGQ_NUM_PAGES - rx` -> `tx - rx <= after_rx.len()`
+            (&after_rx[..(tx - rx)], &after_rx[0..0])
+        } else {
+            // The area is discontiguous.
+            // PANIC: since `tx < rx && before_rx.len() == rx` we have `tx <= before_rx.len()`
+            (after_rx, &before_rx[..tx])
         }
     }
 

-- 
2.52.0