[PATCH AUTOSEL 6.17-5.4] m68k: bitops: Fix find_*_bit() signatures

[Sasha Levin <sashal@kernel.org>]

From: Geert Uytterhoeven <geert@linux-m68k.org>

[ Upstream commit 6d5674090543b89aac0c177d67e5fb32ddc53804 ]

The function signatures of the m68k-optimized implementations of the
find_{first,next}_{,zero_}bit() helpers do not match the generic
variants.

Fix this by changing all non-pointer inputs and outputs to "unsigned
long", and updating a few local variables.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202509092305.ncd9mzaZ-lkp@intel.com/
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: "Yury Norov (NVIDIA)" <yury.norov@gmail.com>
Link: https://patch.msgid.link/de6919554fbb4cd1427155c6bafbac8a9df822c8.1757517135.git.geert@linux-m68k.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

Based on my thorough analysis of this commit, here is my assessment:

## Backport Recommendation: **YES**

### Extensive Analysis

#### 1. **Nature of the Bug**

This commit fixes an **API signature mismatch** between m68k-specific
implementations of `find_{first,next}_{,zero_}bit()` and the generic
variants defined in `include/linux/find.h`.

**Specific Issues Fixed:**
- **Return type mismatch**: m68k returned `int` instead of `unsigned
  long`
- **Parameter type mismatch**: m68k used `int`/`unsigned int` for
  size/offset parameters instead of `unsigned long`
- **Internal variable types**: Updated `res` from `int` to `unsigned
  long` and `words` from `unsigned int` to `unsigned long` for
  consistency

**Code Changes:**
```c
// Before (incorrect):
static inline int find_first_zero_bit(const unsigned long *vaddr,
unsigned size)
{
    int res = 32;
    unsigned int words;
    ...
}

// After (correct):
static inline unsigned long find_first_zero_bit(const unsigned long
*vaddr, unsigned long size)
{
    unsigned long res = 32;
    unsigned long words;
    ...
}
```

#### 2. **Root Cause and Discovery**

**Historical Context:**
- The generic `find_*_bit()` API has used `unsigned long` for return
  values and size/offset parameters since at least **May 2021** (commit
  2cc7b6a44ac21d)
- In **June 2022**, commit 0e862838f2901 unified non-atomic bitops
  prototypes across architectures, but m68k's `find_*_bit()` functions
  were missed
- m68k did update `__fls()` to return `unsigned long` in 2022, but
  `find_*_bit()` was overlooked

**Discovery:**
- Reported by kernel test robot on **September 9, 2025**
- Triggered by the `gpio-mpsse` driver (introduced in v6.13) when
  compiled for m68k with GCC 15.1.0
- Build warning: `warning: format '%ld' expects argument of type 'long
  int', but argument 3 has type 'int' [-Wformat=]`
- The gpio-mpsse driver correctly assumed `find_first_bit()` returns
  `unsigned long` and used `%ld` format specifier

#### 3. **Impact Assessment**

**Build Impact:**
- Causes `-Wformat` warnings with modern compilers (GCC 15.1.0+)
- Breaks W=1 builds (extra warnings enabled)
- Affects m68k-allmodconfig builds

**Runtime Impact:**
- **On m68k (32-bit)**: Both `int` and `long` are 32 bits, so no data
  corruption or truncation at runtime
- **Type safety**: Violates API contract and breaks type safety
  guarantees
- **Future-proofing**: Could cause issues with future compiler
  optimizations or analysis tools

**Affected Code:**
- Any code using these functions with format strings (like gpio-mpsse)
- Any code relying on proper type signatures for static analysis

#### 4. **Fix Quality Assessment**

**Strengths:**
- **Small and focused**: Only changes type signatures, no logic changes
- **Self-contained**: No dependencies on other commits
- **Low risk**: On m68k, `int` and `unsigned long` have identical size
  and alignment
- **Well-tested**: The generic API with these signatures has been used
  successfully across all other architectures since 2021
- **Maintainer approval**: Acked by Yury Norov (NVIDIA), the maintainer
  of bitmap/find_bit subsystem

**Changes Made:**
1. Function return types: `int` → `unsigned long` (4 functions)
2. Size parameters: `unsigned size` → `unsigned long size`
3. Offset parameters: `int offset` → `unsigned long offset`
4. Internal variables: `int res` → `unsigned long res`, `unsigned int
   words` → `unsigned long words`

#### 5. **Consistency with Kernel Standards**

This fix brings m68k in line with:
- The generic API defined in `include/linux/find.h` (lines 385, 179,
  206, 60)
- All other architecture-specific implementations
- The kernel's bitmap subsystem standards established in 2021-2022

#### 6. **Backport Suitability**

**Meets Stable Kernel Criteria:**
✅ **Fixes important bug**: API signature mismatch causing build warnings
✅ **Small and contained**: ~20 lines changed, type-only modifications
✅ **No architectural changes**: Pure signature corrections
✅ **Minimal regression risk**: Same size types on target architecture
✅ **No new features**: Only fixes existing API compliance

**Priority by Kernel Version:**
- **High priority for 6.13+**: Contains gpio-mpsse driver that exposes
  the bug
- **Medium priority for 6.6-6.12**: No immediate triggering code, but
  bug exists
- **Low priority for <6.6**: Older compilers less likely to catch the
  issue, but correctness still matters

**Dependencies:**
None - the fix is self-contained and applies cleanly to any kernel with
the current m68k bitops.h structure (present since at least 2021).

#### 7. **Potential Risks**

**Minimal risks identified:**
- On m68k, `int` and `unsigned long` are both 32-bit, so binary
  compatibility is preserved
- No ABI changes (inline functions)
- No performance impact
- No behavior changes

### Conclusion

This commit is an **excellent candidate for backporting** to stable
kernel trees. It fixes a long-standing API compliance bug with minimal
risk, improves type safety, resolves build warnings with modern
compilers, and aligns m68k with kernel-wide standards. The fix is small,
focused, well-reviewed, and has no dependencies, making it ideal for
stable tree inclusion.

 arch/m68k/include/asm/bitops.h | 25 ++++++++++++++-----------
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/arch/m68k/include/asm/bitops.h b/arch/m68k/include/asm/bitops.h
index 14c64a6f12176..50ec92651d5a5 100644
--- a/arch/m68k/include/asm/bitops.h
+++ b/arch/m68k/include/asm/bitops.h
@@ -350,12 +350,12 @@ static inline bool xor_unlock_is_negative_byte(unsigned long mask,
 #include <asm-generic/bitops/ffz.h>
 #else
 
-static inline int find_first_zero_bit(const unsigned long *vaddr,
-				      unsigned size)
+static inline unsigned long find_first_zero_bit(const unsigned long *vaddr,
+						unsigned long size)
 {
 	const unsigned long *p = vaddr;
-	int res = 32;
-	unsigned int words;
+	unsigned long res = 32;
+	unsigned long words;
 	unsigned long num;
 
 	if (!size)
@@ -376,8 +376,9 @@ static inline int find_first_zero_bit(const unsigned long *vaddr,
 }
 #define find_first_zero_bit find_first_zero_bit
 
-static inline int find_next_zero_bit(const unsigned long *vaddr, int size,
-				     int offset)
+static inline unsigned long find_next_zero_bit(const unsigned long *vaddr,
+					       unsigned long size,
+					       unsigned long offset)
 {
 	const unsigned long *p = vaddr + (offset >> 5);
 	int bit = offset & 31UL, res;
@@ -406,11 +407,12 @@ static inline int find_next_zero_bit(const unsigned long *vaddr, int size,
 }
 #define find_next_zero_bit find_next_zero_bit
 
-static inline int find_first_bit(const unsigned long *vaddr, unsigned size)
+static inline unsigned long find_first_bit(const unsigned long *vaddr,
+					   unsigned long size)
 {
 	const unsigned long *p = vaddr;
-	int res = 32;
-	unsigned int words;
+	unsigned long res = 32;
+	unsigned long words;
 	unsigned long num;
 
 	if (!size)
@@ -431,8 +433,9 @@ static inline int find_first_bit(const unsigned long *vaddr, unsigned size)
 }
 #define find_first_bit find_first_bit
 
-static inline int find_next_bit(const unsigned long *vaddr, int size,
-				int offset)
+static inline unsigned long find_next_bit(const unsigned long *vaddr,
+					  unsigned long size,
+					  unsigned long offset)
 {
 	const unsigned long *p = vaddr + (offset >> 5);
 	int bit = offset & 31UL, res;
-- 
2.51.0