[PATCH v5 10/22] gpu: nova-core: vbios: compute FWSEC-relative Falcon data offset

[Eliot Courtney <ecourtney@nvidia.com>]

Push the computation of the falcon data offset into a helper function.
The subtraction to create the offset should be checked, and by doing
this the check can be folded into the existing check in
`falcon_data_ptr`.

Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Eliot Courtney <ecourtney@nvidia.com>
---
 drivers/gpu/nova-core/vbios.rs | 48 +++++++++++++++++-------------------------
 1 file changed, 19 insertions(+), 29 deletions(-)

diff --git a/drivers/gpu/nova-core/vbios.rs b/drivers/gpu/nova-core/vbios.rs
index cadc6dcffefb..ca101b2b6095 100644
--- a/drivers/gpu/nova-core/vbios.rs
+++ b/drivers/gpu/nova-core/vbios.rs
@@ -846,33 +846,29 @@ fn get_bit_token(&self, token_id: u8) -> Result<BitToken> {
         BitToken::from_id(self, token_id)
     }
 
-    /// Find the Falcon data pointer structure in the [`PciAtBiosImage`].
+    /// Find the Falcon data offset from the start of the FWSEC region.
     ///
-    /// This is just a 4 byte structure that contains a pointer to the Falcon data in the FWSEC
-    /// image.
-    fn falcon_data_ptr(&self) -> Result<u32> {
+    /// The BIT table contains a 4-byte pointer to the Falcon data. Testing shows this pointer
+    /// treats the PCI-AT and FWSEC images as logically contiguous even when an EFI image sits in
+    /// between them, so subtract the PCI-AT image size here to convert it to a FWSEC-relative
+    /// offset.
+    fn falcon_data_offset(&self) -> Result<usize> {
         let token = self.get_bit_token(BIT_TOKEN_ID_FALCON_DATA)?;
-
-        // Make sure we don't go out of bounds
-        if usize::from(token.data_offset) + 4 > self.base.data.len() {
-            return Err(EINVAL);
-        }
-
-        // read the 4 bytes at the offset specified in the token
         let offset = usize::from(token.data_offset);
-        let bytes: [u8; 4] = self.base.data[offset..offset + 4].try_into().map_err(|_| {
-            dev_err!(self.base.dev, "Failed to convert data slice to array\n");
-            EINVAL
-        })?;
 
-        let data_ptr = u32::from_le_bytes(bytes);
+        // Read the 4-byte falcon data pointer at the offset specified in the token.
+        let data = &self.base.data;
+        let (ptr, _) = data
+            .get(offset..)
+            .and_then(u32::from_bytes_copy_prefix)
+            .ok_or(EINVAL)?;
 
-        if (usize::from_safe_cast(data_ptr)) < self.base.data.len() {
-            dev_err!(self.base.dev, "Falcon data pointer out of bounds\n");
-            return Err(EINVAL);
-        }
-
-        Ok(data_ptr)
+        usize::from_safe_cast(ptr)
+            .checked_sub(data.len())
+            .ok_or(EINVAL)
+            .inspect_err(|_| {
+                dev_err!(self.base.dev, "Falcon data pointer out of bounds\n");
+            })
     }
 }
 
@@ -989,15 +985,9 @@ fn setup_falcon_data(
         pci_at_image: &PciAtBiosImage,
         first_fwsec: &FwSecBiosBuilder,
     ) -> Result {
-        let mut offset = usize::from_safe_cast(pci_at_image.falcon_data_ptr()?);
+        let mut offset = pci_at_image.falcon_data_offset()?;
         let mut pmu_in_first_fwsec = false;
 
-        // The falcon data pointer assumes that the PciAt and FWSEC images
-        // are contiguous in memory. However, testing shows the EFI image sits in
-        // between them. So calculate the offset from the end of the PciAt image
-        // rather than the start of it. Compensate.
-        offset -= pci_at_image.base.data.len();
-
         // The offset is now from the start of the first Fwsec image, however
         // the offset points to a location in the second Fwsec image. Since
         // the fwsec images are contiguous, subtract the length of the first Fwsec

-- 
2.54.0