Issue 4237 - Changed initial setting in program_struct

[DavePutz]

Although I cannot find it documented anywhere; looking at _pio_find_offset_for_program() in sdk/src/rp2_common/hardware_pio/pio.c, it is checking if the program origin is >= 0 and, if so, using that origin to look for space. As a result, with the default origin of 0 then pio_can_add_program() will fail on the second attempt to add a program. This limits CP to only adding one program per PIO. This was the cause of issue #4237, where the third call to rp2pio.StateMachine() would fail. The fix simply changes the default from 0 to -1.

[ladyada]

@kevinjwalters please test/review!

[kevinjwalters]

I've not used the C SDK. The documentation does not state what -1 would do: https://raspberrypi.github.io/pico-sdk-doxygen/group__hardware__pio.html#ga3571e3c7c6e645e92b8d438203a911b9 & https://raspberrypi.github.io/pico-sdk-doxygen/structpio__program.html (this is the matching data structure even though it's missing the _t suffix, the quantity is signed making -1 a legal value).

The actual source code can be viewed, I don't know if this matches the SDK in use but it does appear to do some searching for a negative origin value so this looks plausible as a fix: https://github.com/raspberrypi/pico-sdk/blob/786be7524aa3c7c5467faebc54fd7c4db077ae2e/src/rp2_common/hardware_pio/pio.c#L56-L72

[tannewt]

Great find! Thanks @DavePutz