[PATCH AUTOSEL 6.16-5.10] i2c: Force DLL0945 touchpad i2c freq to 100khz

[Sasha Levin <sashal@kernel.org>]

From: "fangzhong.zhou" <myth5@myth5.com>

[ Upstream commit 0b7c9528facdb5a73ad78fea86d2e95a6c48dbc4 ]

This patch fixes an issue where the touchpad cursor movement becomes
slow on the Dell Precision 5560. Force the touchpad freq to 100khz
as a workaround.

Tested on Dell Precision 5560 with 6.14 to 6.14.6. Cursor movement
is now smooth and responsive.

Signed-off-by: fangzhong.zhou <myth5@myth5.com>
[wsa: kept sorting and removed unnecessary parts from commit msg]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

**Backport Status: YES**

This commit should be backported to stable kernel trees for the
following reasons:

## Analysis Supporting Backport:

### 1. **Fixes a Real User-Visible Bug**
The commit fixes a specific hardware malfunction where the touchpad
cursor movement becomes slow on Dell Precision 5560 laptops. This is a
concrete usability issue that affects end users' ability to use their
hardware properly.

### 2. **Minimal and Contained Change**
The change is extremely minimal - it only adds a single line `{
"DLL0945", 0 },` to an existing array
`i2c_acpi_force_100khz_device_ids[]`. This is a purely additive change
that:
- Doesn't modify any existing logic
- Only affects devices with the specific ACPI ID "DLL0945"
- Cannot cause regressions on other hardware

### 3. **Follows Established Pattern**
The code shows this is a well-established pattern for hardware quirks:
- The `i2c_acpi_force_100khz_device_ids[]` array already exists with the
  same purpose
- A nearly identical fix was just added for ELAN06FA touchpad (commit
  bfd74cd1fbc0)
- Similar quirk arrays exist for forcing 400KHz
  (i2c_acpi_force_400khz_device_ids)
- Previous similar fixes have been marked for stable (e.g., commit
  7574c0db2e68 for Silead touchscreens included `Cc: stable@kernel.org`)

### 4. **Hardware-Specific Workaround**
This is a hardware-specific workaround that:
- Only triggers for Dell devices with the DLL0945 touchpad
- Forces I2C bus speed to 100KHz to work around a hardware/firmware
  issue
- Has been tested on the affected hardware (Dell Precision 5560 with
  kernels 6.14 to 6.14.6)

### 5. **No Architecture Changes**
The commit:
- Uses existing infrastructure (the quirk array mechanism)
- Doesn't introduce new features
- Doesn't change any APIs or interfaces
- Simply adds one more device ID to an existing workaround list

### 6. **Low Risk of Regression**
The change has minimal regression risk because:
- It only affects devices with the specific ACPI ID
- The mechanism is already proven with other devices
- The fix is isolated to I2C bus speed negotiation for one specific
  touchpad model
- If the device ID doesn't match, the code path is never executed

### 7. **Consistent with Stable Kernel Rules**
This fix aligns perfectly with stable kernel criteria:
- Fixes a real bug that bothers users (slow touchpad cursor)
- Is obviously correct and tested
- Is small (1 line addition)
- Doesn't add new features
- Fixes only one specific issue

The commit follows the exact same pattern as previous touchpad I2C
frequency quirks that have been successfully backported to stable
kernels, making it a clear candidate for stable tree inclusion.

 drivers/i2c/i2c-core-acpi.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/i2c/i2c-core-acpi.c b/drivers/i2c/i2c-core-acpi.c
index d2499f302b50..f43067f6797e 100644
--- a/drivers/i2c/i2c-core-acpi.c
+++ b/drivers/i2c/i2c-core-acpi.c
@@ -370,6 +370,7 @@ static const struct acpi_device_id i2c_acpi_force_100khz_device_ids[] = {
 	 * the device works without issues on Windows at what is expected to be
 	 * a 400KHz frequency. The root cause of the issue is not known.
 	 */
+	{ "DLL0945", 0 },
 	{ "ELAN06FA", 0 },
 	{}
 };
-- 
2.39.5