drivers/touch/cst816: take the i2c lock around the hardware reset - #1896
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drivers/touch/cst816: take the i2c lock around the hardware reset#1896Mearman wants to merge 1 commit into
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cst816_hw_reset toggles the chip's reset pin with no synchronisation against the bus: touch_sensor_set_enabled is reached synchronously from arbitrary tasks (subscribe/unsubscribe callbacks, the backlight and system-hold toggles) and calls the reset as its first statement, while the system task may be mid-transfer inside prv_read_data clocking the same chip. A chip hit by reset mid-byte stops ACKing and corrupts the in-flight transfer. Take s_i2c_lock inside cst816_hw_reset, the same per-transfer lock the read/write paths use. No caller holds it already -- the only lock-takers are prv_read_data and prv_write_data, neither of which resets -- so the acquisition cannot self-deadlock, and the bootmode/fw-update sequences keep their existing per-operation interleaving. Not unit-testable: the driver has no test target (it sits on real I2C, GPIO and EXTI), and the race is a cross-task timing window against hardware. Validated by call-site audit and a full firmware build. Co-Authored-By: Claude <noreply@anthropic.com> Signed-off-by: Joseph Mearman <joseph@mearman.co.uk>
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Draft — still validating on hardware.
The vaguest of the failures I saw while playing with touch a few weeks ago: very occasional glitched or dropped touch input, never reproducible on demand. Looking at the driver, there is a real unsynchronised window for it.
cst816_hw_reset toggles the chip's reset pin with no synchronisation against the bus: touch_sensor_set_enabled is reached synchronously from arbitrary tasks (subscribe/unsubscribe callbacks, the backlight and system-hold toggles) and calls the reset as its first statement, while the system task may be mid-transfer inside prv_read_data clocking the same chip. A chip hit by reset mid-byte stops ACKing and corrupts the in-flight transfer.
This takes s_i2c_lock inside cst816_hw_reset — the same per-transfer lock the read/write paths use. No caller holds it already (the only lock-takers are prv_read_data and prv_write_data, neither of which resets), so the acquisition cannot self-deadlock, and the bootmode/fw-update sequences keep their existing per-operation interleaving.
No unit test: the driver has no test target (it sits on real I2C, GPIO and EXTI) and the race is a cross-task timing window against hardware. Validated by call-site audit and a full firmware build.