Repository: google/OpenSK
File: third_party/libtock-drivers/src/nfc.rs (Rust)

Code before:

pub struct NfcTag {}

impl NfcTag {
    /// Check the existence of an NFC driver.
    pub fn setup() -> bool {
        syscalls::command(DRIVER_NUMBER, command_nr::CHECK, 0, 0).is_ok()
    }

    pub fn enable_emulation() -> bool {
        NfcTag::emulate(true)
    }

    pub fn disable_emulation() -> bool {
        NfcTag::emulate(false)
    }

    fn emulate(enabled: bool) -> bool {
        syscalls::command(DRIVER_NUMBER, command_nr::EMULATE, enabled as usize, 0).is_ok()
    }

    /// Subscribe to the tag being SELECTED callback.
    pub fn selected() -> bool {
        let is_selected = Cell::new(false);
        let mut is_selected_alarm = || is_selected.set(true);
        let subscription = syscalls::subscribe::<callback::Identity0Consumer, _>(
            DRIVER_NUMBER,
            subscribe_nr::SELECT,
            &mut is_selected_alarm,
        );
        if subscription.is_err() {
            return false;
        }

        util::yieldk_for(|| is_selected.get());
        true
    }

    /// Configure the tag type command.
    pub fn configure(tag_type: u8) -> bool {
        syscalls::command(DRIVER_NUMBER, command_nr::CONFIGURE, tag_type as usize, 0).is_ok()
    }

    /// Set the maximum frame delay value to support transmission with the reader.
    pub fn set_framedelaymax(delay: u32) -> bool {
        syscalls::command(DRIVER_NUMBER, command_nr::FRAMEDELAYMAX, delay as usize, 0).is_ok()
    }

    /// 1. Share with the driver a buffer.
    /// 2. Subscribe to having a successful receive callback.
    /// 3. Issue the request for reception.

Review comment (suggestion):
I'm wondering about the sequence of events here. We subscribe to a callback but without triggering anything with a command, which is unusual with Tock.

How does the driver get notified that it should trigger the callback? Is it triggered just by the subscription (which although unusual is fine)?

I just want to make sure that the application doesn't miss any callback that the kernel would trigger before any subscription has happened.

Code after:
pub struct NfcTag {}

impl NfcTag {
    /// Check the existence of an NFC driver.
    pub fn setup() -> bool {
        syscalls::command(DRIVER_NUMBER, command_nr::CHECK, 0, 0).is_ok()
    }

    pub fn enable_emulation() -> bool {
        NfcTag::emulate(true)
    }

    pub fn disable_emulation() -> bool {
        NfcTag::emulate(false)
    }

    fn emulate(enabled: bool) -> bool {
        syscalls::command(DRIVER_NUMBER, command_nr::EMULATE, enabled as usize, 0).is_ok()
    }

    /// Configure the tag type command.
    pub fn configure(tag_type: u8) -> bool {
        syscalls::command(DRIVER_NUMBER, command_nr::CONFIGURE, tag_type as usize, 0).is_ok()
    }

    /// 1. Share with the driver a buffer.
    /// 2. Subscribe to having a successful receive callback.
    /// 3. Issue the request for reception.
    pub fn receive(buf: &mut [u8; 256]) -> TockResult<RecvOp> {
        let result = syscalls::allow(DRIVER_NUMBER, allow_nr::RECEIVE, buf)?;
        // set callback with 2 arguments, to receive ReturnCode and RX Amount
        let recv_data = Cell::new(None);
        let mut callback = |result, amount| {
            recv_data.set(Some(RecvOp {
                result_code: result,
                recv_amount: amount,
            }))
        };
        let subscription = syscalls::subscribe::<callback::Identity2Consumer, _>(
            DRIVER_NUMBER,
            subscribe_nr::RECEIVE,
            &mut callback,
        )?;
        syscalls::command(DRIVER_NUMBER, command_nr::RECEIVE, 0, 0)?;
        util::yieldk_for(|| recv_data.get().is_some());
        mem::drop(subscription);
        mem::drop(result);
        Ok(recv_data.get().unwrap())
    }

    /// 1. Share with the driver a buffer containing the app's reply.