switch: Fix gyro ZRO handling in IMU sensor updates

[alexeev]

• I confirm that I am the author of this code and release it to the SDL project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Description

This PR fixes gyroscope drift handling for Nintendo Switch controllers by applying the calibrated gyroscope zero-rate offsets when reporting live IMU sensor data.

The change is split into two commits:

1. A small preparatory refactor in LoadIMUCalibration()
2. The actual gyroscope offset fix in SendSensorUpdate()

1. Preparatory refactor

LoadIMUCalibration() reads factory IMU calibration data first, and then attempts to read user calibration data from SPI flash.

The existing user calibration check relies on pIMUScale still pointing at memory that is updated by the next WriteSubcommand() call:

if (WriteSubcommand(ctx, k_eSwitchSubcommandIDs_SPIFlashRead, (uint8_t *)&readParams, sizeof(readParams), &reply) &&
    (pIMUScale[0] | pIMUScale[1] << 8) == 0xA1B2) {
    pIMUScale = reply->spiReadData.rgucReadData;
    ...
}

This works today because the HID read path uses the same backing buffer, but the sequencing is fragile: the magic value is checked before the pointer is explicitly assigned to the newly returned SPI payload.
The first commit makes this dependency explicit by assigning the user calibration payload pointer before checking the magic value. This is intended as a no-behavior-change cleanup that makes the following bug fix safer and easier to review.

2. Gyroscope offset fix

LoadIMUCalibration() already reads the calibrated gyroscope raw offsets from SPI flash. These values represent the controller's zero-rate offset.

Previously, these raw offset values were only used while computing the gyroscope scale factors, and were not retained for live sensor updates. As a result, SendSensorUpdate() multiplied raw gyroscope samples by the scale factor without first subtracting the calibrated zero-rate offset.

This PR stores the calibrated gyroscope offsets and applies the standard IMU conversion:

(raw_value - offset) * scale

The fix preserves the existing axis remapping and sign conventions in SendSensorUpdate().

Testing

Tested using testcontroller with a Nintendo Switch Pro Controller over USB and Bluetooth on MacOS Tahoe 26.5.1

Before this change, a stationary controller reported a persistent gyroscope bias/drift of roughly 8.5 deg/s.

After this change, with the controller stationary on a flat surface, the reported gyroscope values hover close to zero.

 

Existing Issue(s)

No existing GitHub issue found.

Other

I'd appreciate guidance on forward-porting to main.

[slouken]

This looks good! Can you please retarget the main branch? We'll cherry-pick this to release-3.4.x after merging.

[alexeev]

@slouken, sure, re-targeted to the main and tested. Works as expected:

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