Tested on a SECO SBC-B68 and a UDOO X86. The BIOS AML code queries the Embedded Controller over SMBus, respecting the hardware semaphore implementation.
I get this line on kernel log and everything works as expected. [ 7.270172] i801_smbus 0000:00:1f.3: SMBus controller is shared with ACPI AML. This seems safe so far.
Tested with continous use of i2c-tools (i2cdump) with temperature reads in thermal_zone (that triggers AML code).
Tested-by: Ettore Chimenti ek5.chimenti@gmail.com