[PATCH AUTOSEL 6.17-6.12] thermal: intel: selftests: workload_hint: Mask unsupported types

[Sasha Levin <sashal@kernel.org>]

From: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>

[ Upstream commit 0115d063559fa6d25e41751cf455dda40aa2c856 ]

The workload hint may contain some other hints which are not defined.
So mask out unsupported types. Currently only lower 4 bits of workload
type hints are defined.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Link: https://patch.msgid.link/20250828201541.931425-1-srinivas.pandruvada@linux.intel.com
[ rjw: Subject cleanup ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

## BACKPORT RECOMMENDATION: **YES**

### Executive Summary
This commit should be backported to stable kernel trees. It fixes a real
correctness issue in the Intel thermal workload_hint selftest that can
cause false "Invalid workload type index" errors when firmware sets
undefined upper bits in the workload hint value.

### Detailed Analysis

#### Code Change Analysis
The commit adds a single line (`index &= 0x0f;`) at line 147 of
`workload_hint_test.c` that masks the workload type index to only the
lower 4 bits before validation. This occurs in the main polling loop
after reading the workload_type_index from sysfs.

**Location in code flow:**
```c
ret = sscanf(index_str, "%d", &index);
if (ret < 0)
    break;

index &= 0x0f;  // NEW LINE - masks to lower 4 bits

if (index > WORKLOAD_TYPE_MAX_INDEX)
    printf("Invalid workload type index\n");
else
    printf("workload type:%s\n", workload_types[index]);
```

#### Technical Context

1. **Feature Background**: The workload_hint feature was introduced in
   v6.7 (August 2023) for Intel Meteor Lake and newer processors. The
   firmware predicts workload type (idle, battery_life, sustained,
   bursty) and exposes it via MMIO register bits 47:40.

2. **Driver Implementation**: In `processor_thermal_wt_hint.c:73`, the
   kernel driver extracts the workload hint using:
  ```c
  wt = FIELD_GET(SOC_WT, status);  // SOC_WT = GENMASK_ULL(47, 40)
  ```
  This extracts all 8 bits (0-255 possible values) and exposes them to
  userspace via sysfs without any masking.

3. **Specification**: According to the commit message, "Currently only
   lower 4 bits of workload type hints are defined." This means:
   - Bits 0-3: Defined workload types (0=idle, 1=battery_life,
     2=sustained, 3=bursty)
   - Bits 4-7: Undefined/reserved (may be used by firmware for future
     extensions or debugging)

4. **The Bug**: Without the mask, if firmware sets any upper bits (e.g.,
   returns 0x12 = 18):
   - Test reads index=18
   - Check `18 > WORKLOAD_TYPE_MAX_INDEX (3)` triggers
   - Prints "Invalid workload type index"
   - **Incorrect behavior**: The actual workload type is 0x12 & 0x0f = 2
     (sustained)

5. **With the Fix**: Same scenario with mask:
   - Test reads index=18
   - Masks to `18 & 0x0f = 2`
   - Check `2 > 3` passes
   - Prints "workload type:sustained"
   - **Correct behavior**: Properly identifies the workload type

#### Evidence of Real-World Impact

1. **Platform Expansion**: The feature has been extended to Lunar Lake
   and Panther Lake processors (commit b59bd75a4b098, December 2024),
   increasing the likelihood of encountering firmware variations.

2. **Already Selected for Backporting**: The AUTOSEL process has already
   selected this for stable backporting (commit 61100458645b2, signed by
   Sasha Levin on Oct 3, 2025), indicating it meets automated stable
   selection criteria.

3. **Firmware Behavior**: The commit message states "The workload hint
   may contain some other hints which are not defined," indicating this
   is based on actual firmware specification knowledge, not theoretical
   concerns.

#### Stable Kernel Rules Compliance

✅ **Rule 1 - Already in mainline**: Commit 0115d063559fa is in mainline
✅ **Rule 2 - Obviously correct**: Simple bit mask operation, well-
understood
✅ **Rule 3 - Size limit**: 2 lines added, well under 100-line limit
✅ **Rule 4 - Fixes real bug**: Prevents incorrect "Invalid" errors for
valid firmware values
✅ **Rule 5 - User impact**: Affects selftest correctness, important for
validation on stable kernels

#### Risk Assessment

**Risk Level: MINIMAL**

- Single line addition with clear purpose
- No changes to kernel driver code (only selftest)
- No dependencies on other commits
- Cannot cause regressions in kernel functionality
- Makes test more robust against firmware variations
- Simple bitwise AND operation with well-defined behavior

#### Backporting Priority

**Priority: MEDIUM**

- **Not critical**: Doesn't fix a crash, security issue, or data
  corruption
- **Important for correctness**: Ensures selftest provides accurate
  validation
- **Growing relevance**: As feature extends to more Intel platforms,
  correct test behavior becomes more important
- **Low risk, high value**: Simple fix that improves test reliability

### Recommendation Justification

This commit should be backported because:

1. **Fixes incorrect behavior**: The test currently can produce false
   "Invalid" errors for legitimate firmware values
2. **Based on specification**: The fix aligns with the documented bit
   layout (only lower 4 bits defined)
3. **Defensive programming**: Properly handles firmware that sets
   reserved bits
4. **Active feature**: Being extended to new platforms, making correct
   testing increasingly important
5. **Already selected**: AUTOSEL has identified this as worthy of
   backporting
6. **Precedent**: Other selftest fixes are regularly backported to
   stable kernels
7. **Zero risk**: Cannot introduce regressions in kernel functionality

The fix ensures that users running stable kernels can accurately
validate their Intel thermal workload_hint functionality without
encountering spurious "Invalid" messages when firmware follows the
specification by using all 8 bits while only the lower 4 are currently
defined.

 .../selftests/thermal/intel/workload_hint/workload_hint_test.c  | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/testing/selftests/thermal/intel/workload_hint/workload_hint_test.c b/tools/testing/selftests/thermal/intel/workload_hint/workload_hint_test.c
index ba58589a11454..ca2bd03154e4d 100644
--- a/tools/testing/selftests/thermal/intel/workload_hint/workload_hint_test.c
+++ b/tools/testing/selftests/thermal/intel/workload_hint/workload_hint_test.c
@@ -144,6 +144,8 @@ int main(int argc, char **argv)
 			ret = sscanf(index_str, "%d", &index);
 			if (ret < 0)
 				break;
+
+			index &= 0x0f;
 			if (index > WORKLOAD_TYPE_MAX_INDEX)
 				printf("Invalid workload type index\n");
 			else
-- 
2.51.0