[PATCH AUTOSEL 6.17-5.15] tools/cpupower: fix error return value in cpupower_write_sysfs()

[Sasha Levin <sashal@kernel.org>]

From: Kaushlendra Kumar <kaushlendra.kumar@intel.com>

[ Upstream commit 57b100d4cf14276e0340eecb561005c07c129eb8 ]

The cpupower_write_sysfs() function currently returns -1 on
write failure, but the function signature indicates it should
return an unsigned int. Returning -1 from an unsigned function
results in a large positive value rather than indicating
an error condition.

Fix this by returning 0 on failure, which is more appropriate
for an unsigned return type and maintains consistency with typical
success/failure semantics where 0 indicates failure and non-zero
indicates success (bytes written).

Link: https://lore.kernel.org/r/20250828063000.803229-1-kaushlendra.kumar@intel.com
Signed-off-by: Kaushlendra Kumar <kaushlendra.kumar@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

YES

**What It Fixes**
- Corrects an unsigned return error path in `cpupower_write_sysfs()`:
  changes `return -1;` to `return 0;` inside the failure branch `if
  (numwritten < 1)` in `tools/power/cpupower/lib/cpupower.c:56-60`. This
  prevents `-1` from wrapping to a large unsigned value that looks like
  success.
- Aligns write semantics with read: `cpupower_read_sysfs()` already
  returns `0` on failure (`tools/power/cpupower/lib/cpupower.c:30-38`),
  and `cpupower_write_sysfs()` already returns `0` when `open()` fails
  (`tools/power/cpupower/lib/cpupower.c:51-53`).

**User-Visible Impact**
- Current callers interpret `<= 0` as failure. With the buggy `-1`
  (wrapped to large unsigned), failures are silently treated as success.
  Examples:
  - `tools/power/cpupower/utils/helpers/misc.c:83`
  - `tools/power/cpupower/utils/helpers/misc.c:102`
  - `tools/power/cpupower/utils/helpers/misc.c:120`
  - `tools/power/cpupower/utils/helpers/misc.c:289`
- After this fix, these checks correctly detect write failures
  (permission denied, invalid sysfs paths, etc.), improving reliability
  of cpupower operations like setting EPP, turbo boost, or perf bias.

**Scope and Risk**
- One-line change; no API/signature change; no architectural changes.
- Confined to `tools/` (cpupower userspace library). No kernel subsystem
  touched.
- Behavior change is limited to failure paths, converting a silent
  false-success into proper failure detection. Low regression risk and
  consistent with existing read/write patterns.

**Stable Criteria**
- Fixes a real bug affecting users of the cpupower tool (error paths not
  detected).
- Small, contained patch with minimal risk and no new features.
- Consistent semantics across the cpupower lib.
- Although the commit message does not include an explicit Cc: stable,
  the change clearly fits stable backport rules.

In summary, updating `return -1;` to `return 0;` in
`tools/power/cpupower/lib/cpupower.c:59` ensures callers’ `<= 0` checks
work as intended and aligns with existing cpupower I/O conventions. This
is a good candidate for backporting.

 tools/power/cpupower/lib/cpupower.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/power/cpupower/lib/cpupower.c b/tools/power/cpupower/lib/cpupower.c
index ce8dfb8e46abd..d7f7ec6f151c2 100644
--- a/tools/power/cpupower/lib/cpupower.c
+++ b/tools/power/cpupower/lib/cpupower.c
@@ -56,7 +56,7 @@ unsigned int cpupower_write_sysfs(const char *path, char *buf, size_t buflen)
 	if (numwritten < 1) {
 		perror(path);
 		close(fd);
-		return -1;
+		return 0;
 	}
 
 	close(fd);
-- 
2.51.0