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Mesa is Apple's codename for the TouchID sensor. On M1-based systems, it is connected to the SPI bus and communicates via SIO on DMA channels 0x18 and 0x19. The application processors seem to have very little to do with its operation. After power on, the command buffer is encrypted by the SEP and very little useful data can be gleaned from snooping the SIO messages. While the commands are garbled by the SEP, we can see that it has a few recurring themes: * A power on routine involving some sort of calibration, perhaps to get a noise image to subtract from each fingerprint * A polling mode where it is kicked by the kernel and acks if there's no finger on the sensor (runs while macOS waits for a print) * A data transfer mode, where a SIO message is sent to an unmapped EP and the fingerprint scanned into memory. Likely triggered by an interrupt coming off the finger detection ring, but I haven't been able to verify this. Signed-off-by: James Calligeros <jcalligeros99@gmail.com> |
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