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

[Randolph Ha <rha051117@gmail.com>]

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