RTT structure is incorrect for 64-bit targets

[xobs]

The in-memory structures currently used by defmt for RTT are incompatible with the reference code when run in 64-bit platforms.

For example, the header uses usize as the datatype when defining the maximum number of up- and down-channels:

defmt/firmware/defmt-rtt/src/lib.rs

Lines 250 to 251 in ce8c280

	max_up_channels: usize,
	max_down_channels: usize,

However, the upstream library code uses int, which is equivalent to u32 on all modern platforms: https://github.com/SEGGERMicro/RTT/blob/ad6970d813bb12b8a5e34aa94b3a1999f7bd0b6b/RTT/SEGGER_RTT.h#L353-L354

Additionally, all the fields in Channel are defined as usize or AtomicUsize:

defmt/firmware/defmt-rtt/src/channel.rs

Lines 14 to 22 in ce8c280

	pub size: usize,
	/// Written by the target.
	pub write: AtomicUsize,
	/// Written by the host.
	pub read: AtomicUsize,
	/// Channel properties.
	///
	/// Currently, only the lowest 2 bits are used to set the channel mode (see constants below).
	pub flags: AtomicUsize,

However, the official implementation uses unsigned as the type, which is also equivalent to a u32 on all tested platforms: https://github.com/SEGGERMicro/RTT/blob/ad6970d813bb12b8a5e34aa94b3a1999f7bd0b6b/RTT/SEGGER_RTT.h#L340-L343

[jonathanpallant]

Good spot, thanks!

[jonathanpallant]

Do you know of a 64-bit machine that we could perhaps obtain to test 64-bit RTT support with? Ideally one supported in probe-rs as well as with SEGGER J-Link.

[xobs]

I should be able to come up with a Zephyr build for Raspberry Pi 4 that exhibits the problem.

[jonathanpallant]

Ahhh, good call. I saw rust-embedded/rust-raspberrypi-OS-tutorials#232 in my inbox, and I think I have all those pieces ready to go.

Edit: Even better - https://kb.segger.com/Raspberry_Pi_5#Hello_World