Moisture channel 2 (BCM8) conflicts with SPI0 CS0 as of 5.4.51 kernel #1
Description
As of the recently Kernel 5.4.51 it's no longer possible to add_event_detect on an SPI Chip-Select pin while the SPI interface is enabled.
To replicate use the following code snippet on a 5.4.51 Pi vs the previous kernel:
import RPi.GPIO as GPIO
GPIO.setmode(GPIO.BCM)
GPIO.setup(8, GPIO.IN)
def test(pin):
pass
GPIO.add_event_detect(8, edge=GPIO.RISING, callback=test)
It attempts to set up edge detection on SPI's default CE0 pin.
Run this without SPI enabled and it will work fine.
Run it with SPI enabled, and it will fail with "RuntimeError: Failed to add edge detection".
This has changed from previous behaviour and is no doubt the fault of changes as of the 5.x Linux kernel (the new mutually exclusive gpiochip instead of sysfs) and is, furthermore, no doubt fully intended behaviour depite the fact it breaks some potential back-compatibility cases including the use of CS0 to read a moisture channel on the Grow board.
Yes in retrospect using a channel "reserved" by the SPI device in this way was probably never a wise thing to do. But what's done is done.
The fix is simple enough. You must re-allocate the offending chip select channel to a different pin. There's a dtoverlay for this:
dtoverlay=spi0-cs,cs0_pin=14 # Re-assign CS0 from BCM 8 so that Grow can use it
This allocates CS0 to BCM14 (UART transmit) (currently unused by Grow) so that the above code will work in both cases.
This line should be added to the Grow installer to be placed in /boot/config.txt and we should consider moving that pin in a future revision (groan).
We should also communicate the addition of this line to end-uers, since it has the very real possibility of making SPI devices appear not to work if they're in the habit of swapping between HATs. (double groan)
I have raised a counterpart issue against RPi.GPIO to notify others who might have fallen into this trap: https://sourceforge.net/p/raspberry-gpio-python/tickets/184/