Avoid is_mountable calls

[robin-nitrokey]

Filesystem::is_mountable calls Filesystem::mount, checks the result and drops the mounted filesystem. This means that code that first calls is_mountable and then mount on the same storage mounts the storage twice.

nitrokey-3-firmware/components/boards/src/store.rs

Lines 275 to 283 in 6f032b5

	fn init_vfs(
	vfs_storage: &'static mut VolatileStorage,
	vfs_alloc: &'static mut Allocation<VolatileStorage>,
	) -> LfsResult<Filesystem<'static, VolatileStorage>> {
	if !Filesystem::is_mountable(vfs_storage) {
	Filesystem::format(vfs_storage).ok();
	}
	Filesystem::mount(vfs_alloc, vfs_storage)
	}

Instead we should first call Filesystem::mount and handle the error case directly.

[robin-nitrokey]

This should significantly reduce bootup time. With logging enabled (log-semihosting,log-info), removing the is_mountable call reduces the mount time from 1 000 ms to 500 ms and the total bootup time from 2 300 ms to 1 800 ms. Unfortunately, the borrow checker does not allow us to re-try the mount in the error case because of the static lifetime of the returned filesystem. Maybe we can find a workaround.

[sosthene-nitrokey]

We could have a mount_or_format method in littlefs2 ?

I'm not sure I see the borrow checker error. Is this NLL case N°3 again?

[robin-nitrokey]

Yes, we can do it in littlefs2 but not directly in the firmware.