[PATCH] Force ELAN06FA touchpad I2C bus freq to 100KHz

Randolph Ha
2025-01-11T15:58:18Z

When a 400KHz freq is used on this model of ELAN touchpad in Linux,
excessive smoothing (similar to when the touchpad's firmware detects
a noisy signal) is sometimes applied. As some devices' (e.g, Lenovo
V15 G4) ACPI tables do not specify a frequency for this device and
some I2C busses (e.g, Designware I2C) default to a 400KHz freq,
force the speed to 100KHz as a workaround.

This problem may be related to the default HCNT/LCNT values given by
some busses' drivers, because they are also not specified in the
aforementioned devices' ACPI tables, and because the device works at
what appears to be a 400KHz frequency in Windows without issues.

Signed-off-by: Randolph Ha <rha051117@gmail.com>
---
drivers/i2c/i2c-core-acpi.c | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)

diff --git a/drivers/i2c/i2c-core-acpi.c b/drivers/i2c/i2c-core-acpi.c
index 14ae0cfc325e..6f1c2377db0a 100644
--- a/drivers/i2c/i2c-core-acpi.c
+++ b/drivers/i2c/i2c-core-acpi.c
@@ -355,6 +355,24 @@ static const struct acpi_device_id i2c_acpi_force_400khz_device_ids[] = {
{}
};

+static const struct acpi_device_id i2c_acpi_force_100khz_device_ids[] = {
+ /*
+ * When a 400KHz freq is used on this model of ELAN touchpad in Linux,
+ * excessive smoothing (similar to when the touchpad's firmware detects
+ * a noisy signal) is sometimes applied. As some devices' (e.g, Lenovo
+ * V15 G4) ACPI tables do not specify a frequency for this device and
+ * some I2C busses (e.g, Designware I2C) default to a 400KHz freq,
+ * force the speed to 100KHz as a workaround.
+ *
+ * This problem may be related to the default HCNT/LCNT values given by
+ * some busses' drivers, because they are also not specified in the
+ * aforementioned devices' ACPI tables, and because the device works at
+ * what appears to be a 400KHz frequency in Windows without issues.
+ */
+ { "ELAN06FA", 0 },
+ {}
+};
+
static acpi_status i2c_acpi_lookup_speed(acpi_handle handle, u32 level,
void *data, void **return_value)
{
@@ -373,6 +391,9 @@ static acpi_status i2c_acpi_lookup_speed(acpi_handle handle, u32 level,
if (acpi_match_device_ids(adev, i2c_acpi_force_400khz_device_ids) == 0)
lookup->force_speed = I2C_MAX_FAST_MODE_FREQ;

+ if (acpi_match_device_ids(adev, i2c_acpi_force_100khz_device_ids) == 0)
+ lookup->force_speed = I2C_MAX_STANDARD_MODE_FREQ;
+
return AE_OK;
}

--
2.47.1