embassy/embassy-rp/src/bootsel.rs

//! Boot Select button
//!
//! The RP2040 rom supports a BOOTSEL button that is used to enter the USB bootloader
//! if held during reset. To avoid wasting GPIO pins, the button is multiplexed onto
//! the CS pin of the QSPI flash, but that makes it somewhat expensive and complicated
//! to utilize outside of the rom's bootloader.
//!
//! This module provides functionality to poll BOOTSEL from an embassy application.

use crate::flash::in_ram;

impl crate::peripherals::BOOTSEL {
    /// Polls the BOOTSEL button. Returns true if the button is pressed.
    ///
    /// Polling isn't cheap, as this function waits for core 1 to finish it's current
    /// task and for any DMAs from flash to complete
    pub fn is_pressed(&mut self) -> bool {
        let mut cs_status = Default::default();

        unsafe { in_ram(|| cs_status = ram_helpers::read_cs_status()) }.expect("Must be called from Core 0");

        // bootsel is active low, so invert
        !cs_status.infrompad()
    }
}

mod ram_helpers {
    use rp_pac::io::regs::GpioStatus;

    /// Temporally reconfigures the CS gpio and returns the GpioStatus.

    /// This function runs from RAM so it can disable flash XIP.
    ///
    /// # Safety
    ///
    /// The caller must ensure flash is idle and will remain idle.
    /// This function must live in ram. It uses inline asm to avoid any
    /// potential calls to ABI functions that might be in flash.
    #[inline(never)]
    #[link_section = ".data.ram_func"]
    #[cfg(target_arch = "arm")]
    pub unsafe fn read_cs_status() -> GpioStatus {
        let result: u32;

        // Magic value, used as both OEOVER::DISABLE and delay loop counter
        let magic = 0x2000;

        core::arch::asm!(
            ".equiv GPIO_STATUS, 0x0",
            ".equiv GPIO_CTRL,   0x4",

            "ldr {orig_ctrl}, [{cs_gpio}, $GPIO_CTRL]",

            // The BOOTSEL pulls the flash's CS line low though a 1K resistor.
            // this is weak enough to avoid disrupting normal operation.
            // But, if we disable CS's output drive and allow it to float...
            "str {val}, [{cs_gpio}, $GPIO_CTRL]",

            // ...then wait for the state to settle...
            "1:", // ~4000 cycle delay loop
            "subs {val}, #8",
            "bne 1b",

            // ...we can read the current state of bootsel
            "ldr {val}, [{cs_gpio}, $GPIO_STATUS]",

            // Finally, restore CS to normal operation so XIP can continue
            "str {orig_ctrl}, [{cs_gpio}, $GPIO_CTRL]",

            cs_gpio = in(reg) rp_pac::IO_QSPI.gpio(1).as_ptr(),
            orig_ctrl = out(reg) _,
            val = inout(reg) magic => result,
            options(nostack),
        );

        core::mem::transmute(result)
    }

    #[cfg(not(target_arch = "arm"))]
    pub unsafe fn read_cs_status() -> GpioStatus {
        unimplemented!()
    }
}