From: Romain Gantois romain.gantois@bootlin.com
[ Upstream commit d511206dc7443120637efd9cfa3ab06a26da33dd ]
The regulator_set_voltage() function may exhibit unexpected behavior if the target regulator has a maximum voltage step constraint. With such a constraint, the regulator core may clamp the requested voltage to a lesser value, to ensure that the voltage delta stays under the specified limit.
This means that the resulting regulator voltage depends on the current voltage, as well as the requested range, which invalidates the assumption that a repeated request for a specific voltage range will amount to a noop.
Considering the case of a regulator with a maximum voltage step constraint of 1V:
initial voltage: 2.5V
consumer requests 4V expected result: 3.5V resulting voltage: 3.5V
consumer requests 4V again expected result: 4V actual result: 3.5V
Correct this by repeating attempts to balance the regulator voltage until the result converges.
Signed-off-by: Romain Gantois romain.gantois@bootlin.com Link: https://patch.msgid.link/20250718-regulator-stepping-v2-1-e28c9ac5d54a@bootl... Signed-off-by: Mark Brown broonie@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org ---
LLM Generated explanations, may be completely bogus:
**Backport Status: YES**
This commit fixes a legitimate bug in the voltage regulator subsystem that affects users with stepped voltage regulators. Here's my extensive analysis:
## Bug Description and Impact
The commit addresses a bug where regulators with `max_uV_step` constraints fail to reach their target voltage when the same voltage range is requested multiple times. The bug manifests as follows:
1. When a regulator has a maximum voltage step constraint (e.g., 1V), the core limits each voltage change to stay within this step 2. The optimization at line 3814 (`if (voltage->min_uV == min_uV && voltage->max_uV == max_uV) goto out;`) assumes repeated requests are no-ops 3. This creates a situation where requesting 4V twice from 2.5V only reaches 3.5V, not the desired 4V
## Code Analysis
The fix adds a retry mechanism specifically for stepped regulators:
1. **New helper function** `regulator_get_voltage_delta()` (lines 3800-3808): Calculates the absolute difference between current and target voltage 2. **Retry loop** (lines 3865-3893): After the initial voltage setting, if `max_uV_step` is configured, it: - Checks if we've reached the target voltage (delta > 0) - Repeatedly calls `regulator_balance_voltage()` until convergence - Includes convergence protection to avoid infinite loops (line 3888)
## Why This Is a Good Backport Candidate
1. **Fixes a real bug**: Users with stepped voltage regulators cannot reach target voltages, potentially causing system instability or device malfunction 2. **Minimal invasive changes**: The fix is well-contained within `regulator_set_voltage_unlocked()` and only affects regulators with `max_uV_step` constraints 3. **No API/ABI changes**: Only internal implementation changes, no external interfaces modified 4. **Clear regression potential**: Low risk as the new code only executes for regulators with `max_uV_step` set 5. **Safety checks included**: The convergence check prevents infinite loops (`if (new_delta - delta > rdev->constraints->max_uV_step)`)
## Specific Code References
- The bug is in the optimization at drivers/regulator/core.c:3814-3815 - The fix adds retry logic at drivers/regulator/core.c:3865-3893 - Only affects regulators where `rdev->constraints->max_uV_step > 0` - The existing `regulator_limit_voltage_step()` function already handles the step limiting logic
This is exactly the type of fix that belongs in stable: it addresses a specific functional bug without introducing new features or architectural changes.
drivers/regulator/core.c | 43 +++++++++++++++++++++++++++++++++++++++- 1 file changed, 42 insertions(+), 1 deletion(-)
diff --git a/drivers/regulator/core.c b/drivers/regulator/core.c index 90629a756693..dbb14f79e44e 100644 --- a/drivers/regulator/core.c +++ b/drivers/regulator/core.c @@ -3797,6 +3797,16 @@ static int _regulator_do_set_suspend_voltage(struct regulator_dev *rdev, return 0; }
+static int regulator_get_voltage_delta(struct regulator_dev *rdev, int uV) +{ + int current_uV = regulator_get_voltage_rdev(rdev); + + if (current_uV < 0) + return current_uV; + + return abs(current_uV - uV); +} + static int regulator_set_voltage_unlocked(struct regulator *regulator, int min_uV, int max_uV, suspend_state_t state) @@ -3804,8 +3814,8 @@ static int regulator_set_voltage_unlocked(struct regulator *regulator, struct regulator_dev *rdev = regulator->rdev; struct regulator_voltage *voltage = ®ulator->voltage[state]; int ret = 0; + int current_uV, delta, new_delta; int old_min_uV, old_max_uV; - int current_uV;
/* If we're setting the same range as last time the change * should be a noop (some cpufreq implementations use the same @@ -3852,6 +3862,37 @@ static int regulator_set_voltage_unlocked(struct regulator *regulator, voltage->max_uV = old_max_uV; }
+ if (rdev->constraints->max_uV_step > 0) { + /* For regulators with a maximum voltage step, reaching the desired + * voltage might take a few retries. + */ + ret = regulator_get_voltage_delta(rdev, min_uV); + if (ret < 0) + goto out; + + delta = ret; + + while (delta > 0) { + ret = regulator_balance_voltage(rdev, state); + if (ret < 0) + goto out; + + ret = regulator_get_voltage_delta(rdev, min_uV); + if (ret < 0) + goto out; + + new_delta = ret; + + /* check that voltage is converging quickly enough */ + if (new_delta - delta > rdev->constraints->max_uV_step) { + ret = -EWOULDBLOCK; + goto out; + } + + delta = new_delta; + } + } + out: return ret; }