atmel-samd: Fix CDC #294
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atmel-samd: Fix CDC #294
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CONF_USB_COMPOSITE_CDC_ACM_EN before including ASF4 files.
dhalbert
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Sep 29, 2017
jepler
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Nov 3, 2024
Since the very beginning, the stm32 port (first called stm, then stmhal now stm32) has had a special keyboard interrupt feature which works by using PendSV to break out of any running code. This preemptive ctrl-C was added long ago in commit 01156d5. The stm32 port still uses that code, and current does this: - If ctrl-C is received on UART or USB then `mp_sched_keyboard_interrupt()` is called (like all other ports) to set a flag for the VM to see, and then the VM (or any loop calling `mp_handle_pending(true)`) will eventually handle the `KeyboardInterrupt` exception, raising it via NLR. - If another ctrl-C is received while the existing scheduled keyboard interrupt is still pending (ie the VM has not yet processed it) then a special hard NLR jump will activate, that preempts the calling code. Within the PendSV interrupt the stack is adjusted and an NLR jump is made to the most recent `nlr_push()` location. This is like a normal NLR except it is called from an interrupt context and completely annihilates the code that was interrupted by the IRQ. The reason for the preemptive interrupt was to handle ctrl-C before the VM was able to handle it. Eventually a mechanism (that's in use today by all ports) was added to the VM and runtime to be able to check for pending interrupts. Then the stm32 port was updated to use this mechanism, with a fallback to the old preemptive way if a second ctrl-C was received (without the first one being processed). This preemptive NLR jump is problematic because it can interrupt long-running instructions (eg store multiple, usually used at the end of a function to restore registers and return). If such an instruction is interrupted the CPU remembers that with some flags, and can resume the long-running instruction when the interrupt finishes. But the preemptive NLR does a long jump to different code at thread level and so the long-running interrupt is never resumed. This leads to a CPU fault. This fault has been previously reported in issues adafruit#3807 and adafruit#3842 (see also issue adafruit#294). It's now possible to easily reproduce this problem, since commit 69c25ea. Running the test suite over and over again on any stm32 board will eventually crash the board (it can happen on a PYBv1.x, but it happens more regularly on PYBD-SF2/6). The point is, a skipped test now soft resets the board and so the board must run `boot.py` again. The test runner may then interrupt the execution of `boot.py` with the double-ctrl-C that it sends (in `tools/pyboard.py`, `enter_raw_repl()`) in order to get the board into a known good state for the next test. If the timing is right, this can trigger the preemptive PendSV in an unfortunate location and hard fault the board. The fix in this commit is to just remove the preemptive NLR jump feature. No other port has this feature and it's not needed, ctrl-C works very well on those ports. Preemptive NLR jump is a very dangerous thing (eg it may interrupt and break out of an external SPI flash operation when reading code from a filesystem) and is obviously buggy. With this commit, stm32 borads no longer hard fault when running the test suite (but it does leave an issue, the tests can still interrupt `boot.py` with a single ctrl-C; that will be fixed separately). An alternative to this commit would be to clear the CPU state for the long-running instruction as suggested in issue adafruit#3842. But it's much simpler to just remove this code, which is now unnecessary and can have other problems as per issue adafruit#294. Signed-off-by: Damien George <damien@micropython.org>
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by making sure we define CONF_USB_COMPOSITE_CDC_ACM_EN before including ASF4 files.