[PATCH AUTOSEL 5.4 02/20] i2c: designware: Invoke runtime suspend on quick slave re-registration

[PATCH AUTOSEL 5.4 02/20] i2c: designware: Invoke runtime suspend on quick slave re-registration

[Sasha Levin]

From: Tan En De <ende.tan@starfivetech.com>

[ Upstream commit 2fe2b969d911a09abcd6a47401a3c66c38a310e6 ]

Replaced pm_runtime_put() with pm_runtime_put_sync_suspend() to ensure
the runtime suspend is invoked immediately when unregistering a slave.
This prevents a race condition where suspend was skipped when
unregistering and registering slave in quick succession.

For example, consider the rapid sequence of
`delete_device -> new_device -> delete_device -> new_device`.
In this sequence, it is observed that the dw_i2c_plat_runtime_suspend()
might not be invoked after `delete_device` operation.

This is because after `delete_device` operation, when the
pm_runtime_put() is about to trigger suspend, the following `new_device`
operation might race and cancel the suspend.

If that happens, during the `new_device` operation,
dw_i2c_plat_runtime_resume() is skipped (since there was no suspend), which
means `i_dev->init()`, i.e. i2c_dw_init_slave(), is skipped.
Since i2c_dw_init_slave() is skipped, i2c_dw_configure_fifo_slave() is
skipped too, which leaves `DW_IC_INTR_MASK` unconfigured. If we inspect
the interrupt mask register using devmem, it will show as zero.

Example shell script to reproduce the issue:
```
  #!/bin/sh

  SLAVE_LADDR=0x1010
  SLAVE_BUS=13
  NEW_DEVICE=/sys/bus/i2c/devices/i2c-$SLAVE_BUS/new_device
  DELETE_DEVICE=/sys/bus/i2c/devices/i2c-$SLAVE_BUS/delete_device

  # Create initial device
  echo slave-24c02 $SLAVE_LADDR > $NEW_DEVICE
  sleep 2

  # Rapid sequence of
  # delete_device -> new_device -> delete_device -> new_device
  echo $SLAVE_LADDR > $DELETE_DEVICE
  echo slave-24c02 $SLAVE_LADDR > $NEW_DEVICE
  echo $SLAVE_LADDR > $DELETE_DEVICE
  echo slave-24c02 $SLAVE_LADDR > $NEW_DEVICE

  # Using devmem to inspect IC_INTR_MASK will show as zero
```

Signed-off-by: Tan En De <ende.tan@starfivetech.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Link: https://lore.kernel.org/r/20250412023303.378600-1-ende.tan@starfivetech.com
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

**YES** This commit should be backported to stable kernel trees. Here's
my extensive analysis: ## Critical Bug Fix Analysis ### 1. **Nature of
the Bug** This commit fixes a **race condition** that causes I2C slave
functionality to completely break. The problem occurs during rapid slave
device registration/unregistration sequences where: - `pm_runtime_put()`
is asynchronous and may not complete suspend before the next
`pm_runtime_get_sync()` - If suspend doesn't happen, runtime resume is
skipped - Runtime resume is where `i2c_dw_init_slave()` and
`i2c_dw_configure_fifo_slave()` restore critical hardware configuration
- When these initialization functions are skipped, the interrupt mask
register (`DW_IC_INTR_MASK`) remains zero, disabling all interrupts ###
2. **Impact on Users** This is a **high-impact functional bug** that: -
Completely breaks I2C slave functionality in affected scenarios -
Affects real-world use cases (rapid device creation/deletion as shown in
the shell script) - Leaves hardware in a non-functional state requiring
manual intervention - Results in silent failures where the slave appears
registered but cannot handle transactions ### 3. **Code Change
Analysis** The fix is **minimal and surgical**: ```c -
pm_runtime_put(dev->dev); + pm_runtime_put_sync_suspend(dev->dev); ```
This single-line change: - Replaces asynchronous `pm_runtime_put()` with
synchronous `pm_runtime_put_sync_suspend()` - Ensures runtime suspend
happens immediately before unregistration completes - Eliminates the
race window where suspend could be skipped - Has **no functional side
effects** beyond the intended timing change ### 4. **Risk Assessment**
**Very low risk** for several reasons: - **Single line change** with
clear, well-understood semantics - **Conservative fix** - makes behavior
more predictable, not fundamentally different - **Same end result** -
both functions eventually suspend the device, just with different timing
- **No new code paths** - uses existing, well-tested runtime PM
infrastructure - **Maintains all existing error handling and power
management policies** ### 5. **Stable Tree Criteria Compliance** ✅
**Important bugfix** - Fixes complete loss of I2C slave functionality ✅
**Minimal risk** - Single line change with well-understood behavior ✅
**No new features** - Pure bug fix with no feature additions ✅ **No
architectural changes** - Uses existing runtime PM APIs ✅ **Confined to
subsystem** - Only affects i2c-designware slave mode ✅ **Clear
reproduction case** - Includes shell script demonstrating the problem
### 6. **Comparison with Similar Commits** Looking at the provided
examples: - **Similar Commit #3 (Status: YES)**: Also fixed a race
condition in slave mode with `synchronize_irq()` - this shows precedent
for backporting race condition fixes in this driver - **Similar Commits
#1,2,4,5 (Status: NO)**: Were cleanup, optimization, or architectural
changes rather than critical bug fixes ### 7. **Technical Validation**
The commit demonstrates understanding of the underlying issue: - **Root
cause identified**: Async suspend being canceled by subsequent
operations - **Mechanism explained**: Missing initialization during
runtime resume - **Hardware impact documented**: Interrupt mask register
left unconfigured - **Reproduction provided**: Clear test case showing
the failure ### 8. **Maintainer Confidence** The commit has strong
technical validation: - **Acked-by: Jarkko Nikula** - A key
i2c-designware maintainer - **Detailed commit message** with technical
explanation and reproduction steps - **Clean implementation** using
standard kernel APIs ## Conclusion This commit fixes a critical race
condition that completely breaks I2C slave functionality with minimal
risk and clear precedent for backporting similar fixes. It meets all
stable tree criteria for an important, low-risk bugfix that should be
available to users running stable kernels.

 drivers/i2c/busses/i2c-designware-slave.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/i2c/busses/i2c-designware-slave.c b/drivers/i2c/busses/i2c-designware-slave.c
index f5f001738df5e..57e67962a602f 100644
--- a/drivers/i2c/busses/i2c-designware-slave.c
+++ b/drivers/i2c/busses/i2c-designware-slave.c
@@ -96,7 +96,7 @@ static int i2c_dw_unreg_slave(struct i2c_client *slave)
 	dev->disable(dev);
 	synchronize_irq(dev->irq);
 	dev->slave = NULL;
-	pm_runtime_put(dev->dev);
+	pm_runtime_put_sync_suspend(dev->dev);
 
 	return 0;
 }
-- 
2.39.5