Can't use GPIO_B2

[pojiro]

We cannot use it. Because GPIO_B2 is already used for UART4_RX, ttySTM0.

---

iex(12)> {:ok, gpio} = Circuits.GPIO.open(18, :output)
** (MatchError) no match of right hand side value: {:error, :export_failed}

iex(29)> {:ok, gpio} = Circuits.GPIO.open(18, :input)
** (MatchError) no match of right hand side value: {:error, :export_failed}

The serial output at this time is as follows

[  628.569771] stm32mp157-pinctrl soc:pin-controller@50002000: pin PB2 already requested by 40010000.serial; cannot claim for GPIOB:18
[  628.580319] stm32mp157-pinctrl soc:pin-controller@50002000: pin-18 (GPIOB:18) status -22
Error writing '18' to /sys/class/gpio/export

Going back to the boot log, we find the following, which are already in use

[    2.069453] stm32-usart 40010000.serial: rx dma alloc failed
[    2.069488] stm32-usart 40010000.serial: interrupt mode used for rx (no dma)
[    2.069508] stm32-usart 40010000.serial: tx dma alloc failed
[    2.069525] stm32-usart 40010000.serial: interrupt mode used for tx (no dma)
[    2.069565] 40010000.serial: ttySTM0 at MMIO 0x40010000 (irq = 31, base_baud = 4000000) is a stm32-usart
[    3.170536] printk: console [ttySTM0] enabled

40010000.serial is uart4 on the schematic. We can see that GPIO_B2 = QSPI_CLK = UART4_RX.