This fixes 2 issues where STM32 BXCAN would hang

1. If one received frames under an `interrupt_free` section, in my case
   `init` in RTIC, the RX IRQ will fire and clear it's enable bit after
   `interrupt_free` is complete. There is no frame to read so RX is now
   unconditionally disabled forever.
2. On clearing of RX IRQ, TX stops silently. This happens due to the use
   of `write` instead of `modify` when modifying IRQ enable bits.

Solution 1: Enable RX IRQs on every call to `try_read` that return no
data. This solution also solves the issue of very delayed handling of
the RX IRQ which would cause the same issue.

Solution 2: Use `modify` instead of `write`.

1 files changed, 7 insertions, 3 deletions

diff --git a/embassy-stm32/src/can/bxcan/mod.rs b/embassy-stm32/src/can/bxcan/mod.rs
index baa4bee79..19f1cea29 100644
--- a/embassy-stm32/src/can/bxcan/mod.rs
+++ b/embassy-stm32/src/can/bxcan/mod.rs
@@ -893,7 +893,7 @@ impl RxMode {
                     RxFifo::Fifo0 => 0usize,
                     RxFifo::Fifo1 => 1usize,
                 };
-                T::regs().ier().write(|w| {
+                T::regs().ier().modify(|w| {
                     w.set_fmpie(fifo_idx, false);
                 });
                 waker.wake();
@@ -936,18 +936,22 @@ impl RxMode {
             Self::NonBuffered(_) => {
                 let registers = &info.regs;
                 if let Some(msg) = registers.receive_fifo(RxFifo::Fifo0) {
-                    registers.0.ier().write(|w| {
+                    registers.0.ier().modify(|w| {
                         w.set_fmpie(0, true);
                     });
                     Ok(msg)
                 } else if let Some(msg) = registers.receive_fifo(RxFifo::Fifo1) {
-                    registers.0.ier().write(|w| {
+                    registers.0.ier().modify(|w| {
                         w.set_fmpie(1, true);
                     });
                     Ok(msg)
                 } else if let Some(err) = registers.curr_error() {
                     Err(TryReadError::BusError(err))
                 } else {
+                    registers.0.ier().modify(|w| {
+                        w.set_fmpie(0, true);
+                        w.set_fmpie(1, true);
+                    });
                     Err(TryReadError::Empty)
                 }
             }