The write size for the concatenated flashes must be the same

Error message

The write size for the concatenated flashes must be the same

What it means

The const fn `get_write_size` panics when the two flashes combined by `ConcatFlash::new` declare different WRITE_SIZE constants. A concatenated flash exposes a single write granularity, so mismatched write sizes are rejected. As a compile-time (const-eval) panic, it typically surfaces when instantiating the ConcatFlash type.

Solutions

Choose two flashes whose WRITE_SIZE constants match
Wrap the odd flash in an adapter normalizing WRITE_SIZE to the shared value
Buffer writes so the effective write size is identical (e.g. page-buffer wrapper)
Review driver release notes for changed WRITE_SIZE constants after an update

Example fix

// before
type Boot = ConcatFlash<InternalFlash, SpiNor>; // WRITE_SIZE 256 vs 1
// after
type Boot = ConcatFlash<InternalFlash, NorWrapper<256, SpiNor>>;

Defensive patterns

Strategy: validation

Validate before calling

const fn write_sizes_match<A: NorFlash, B: NorFlash>() -> bool { A::WRITE_SIZE == B::WRITE_SIZE }
const _: () = assert!(write_sizes_match::<MyFlashA, MyFlashB>(), "WRITE_SIZE mismatch for ConcatFlash");

Prevention

Confirm WRITE_SIZE equality before ConcatFlash::new
Use a page-buffer wrapper to present a uniform write granularity
Pin and review driver versions so WRITE_SIZE constants don't change silently
Encode flash constants as board-level const items reused everywhere

When it happens

Trigger: `ConcatFlash::new(a, b)` where `First::WRITE_SIZE != Second::WRITE_SIZE`, evaluated by the NorFlash/AsyncNorFlash WRITE_SIZE computation.

Common situations: Mixing internal MCU flash (WRITE_SIZE 8/256) with SPI NOR flash (WRITE_SIZE 1 or 256); switching one flash to a chip with different page-write granularity; firmware version change that altered a driver's WRITE_SIZE.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of embassy-rs/embassy@463a07b963 (2026-09-10). Data as JSON: /api/errors/47144fd6e64d7e4d. Report an issue: GitHub.

Appendix: source

Thrown at embassy-embedded-hal/src/flash/concat_flash.rs:27

pub struct ConcatFlash<First, Second>(First, Second);

impl<First, Second> ConcatFlash<First, Second> {
    /// Create a new flash that concatenates two consecutive flashes.
    pub fn new(first: First, second: Second) -> Self {
        Self(first, second)
    }
}

const fn get_read_size(first_read_size: usize, second_read_size: usize) -> usize {
    if first_read_size != second_read_size {
        panic!("The read size for the concatenated flashes must be the same");
    }
    first_read_size
}

const fn get_write_size(first_write_size: usize, second_write_size: usize) -> usize {
    if first_write_size != second_write_size {
        panic!("The write size for the concatenated flashes must be the same");
    }
    first_write_size
}

const fn get_max_erase_size(first_erase_size: usize, second_erase_size: usize) -> usize {
    let max_erase_size = if first_erase_size > second_erase_size {
        first_erase_size
    } else {
        second_erase_size
    };
    if max_erase_size % first_erase_size != 0 || max_erase_size % second_erase_size != 0 {
        panic!("The erase sizes for the concatenated flashes must have have a gcd equal to the max erase size");
    }
    max_erase_size
}

impl<First, Second, E> ErrorType for ConcatFlash<First, Second>
where

View on GitHub (pinned to 463a07b963)