[PATCH v2 4/4] gpu: nova-core: use `checked_ilog2` to emulate `fls`

[Alexandre Courbot <acourbot@nvidia.com>]

Rust's `checked_ilog2` is in effect equivalent to the C `fls` operation,
with the exception that its result is zero-indexed. This means we don't
have a good basis to introduce an equivalent of `fls` on our own.

Convert the relevant Nova code to use `checked_ilog2`, and remove the
corresponding TODO item.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 Documentation/gpu/nova/core/todo.rst      | 14 --------------
 drivers/gpu/nova-core/falcon/hal/ga102.rs |  4 ++--
 2 files changed, 2 insertions(+), 16 deletions(-)

diff --git a/Documentation/gpu/nova/core/todo.rst b/Documentation/gpu/nova/core/todo.rst
index 8fdb5bced3460a3971699df79ffa2c69f84b2735..01dfa858d11fe377c345b463742c13c37878e334 100644
--- a/Documentation/gpu/nova/core/todo.rst
+++ b/Documentation/gpu/nova/core/todo.rst
@@ -141,20 +141,6 @@ Features desired before this happens:
 | Complexity: Advanced
 | Contact: Alexandre Courbot
 
-Numerical operations [NUMM]
----------------------------
-
-Nova uses integer operations that are not part of the standard library (or not
-implemented in an optimized way for the kernel). These include:
-
-- The "Find Last Set Bit" (`fls` function of the C part of the kernel)
-  operation.
-
-A `num` core kernel module is being designed to provide these operations.
-
-| Complexity: Intermediate
-| Contact: Alexandre Courbot
-
 Delay / Sleep abstractions [DLAY]
 ---------------------------------
 
diff --git a/drivers/gpu/nova-core/falcon/hal/ga102.rs b/drivers/gpu/nova-core/falcon/hal/ga102.rs
index 52c33d3f22a8e920742b45940c346c47fdc70e93..430a511aa1f85477690e78cdc1104f0e0097b0e4 100644
--- a/drivers/gpu/nova-core/falcon/hal/ga102.rs
+++ b/drivers/gpu/nova-core/falcon/hal/ga102.rs
@@ -69,8 +69,8 @@ fn signature_reg_fuse_version_ga102(
     let reg_fuse_version =
         bar.read32(reg_fuse_base + ((ucode_id - 1) as usize * core::mem::size_of::<u32>()));
 
-    // TODO[NUMM]: replace with `last_set_bit` once it lands.
-    Ok(u32::BITS - reg_fuse_version.leading_zeros())
+    // The original C code performs a `fls`; this is equivalent.
+    Ok(reg_fuse_version.checked_ilog2().map_or(0, |v| v + 1))
 }
 
 fn program_brom_ga102<E: FalconEngine>(bar: &Bar0, params: &FalconBromParams) -> Result {

-- 
2.50.1