What causes "alloc_endpoint_out failed" in embassy-rs/embassy?

Reduce the number of OUT endpoints in the class/composite configuration

alloc_endpoint_out failed

Error message

alloc_endpoint_out failed

What it means

The USB builder's alloc_endpoint_out forwards to the driver's endpoint allocator and expects success via `.expect`. The driver could not allocate an OUT endpoint with the requested type/address/packet size, usually because all hardware OUT endpoints are in use or the request is unsupported. This aborts the descriptor-building task.

Solutions

Reduce the number of OUT endpoints in the class/composite configuration
Pass `None` for ep_addr to let the driver pick a free endpoint
Lower `max_packet_size` or use an endpoint type the hardware supports
Check the driver/peripheral endpoint budget (e.g. OTG-FS supports fewer endpoints than HS) and adjust device selection

Example fix

// before
let ep = builder.endpoint_out(EndpointType::Bulk, Some(EndpointAddress::from_parts(0x01)), 512, 0);
// after
let ep = builder.endpoint_out(EndpointType::Bulk, None, 64, 0);

Defensive patterns

Strategy: validation

Validate before calling

// before building: count OUT endpoints in your classes
// and ensure count <= driver max OUT endpoints

Prevention

Budget endpoints per interface before configuring composite devices
Pass None for endpoint addresses to avoid collisions
Match packet sizes to bus speed limits (64 bytes for FS)
Check chip variant endpoint capacity when switching targets

When it happens

Trigger: Calling `endpoint_out(ep_type, ep_addr, max_packet_size, interval_ms)` on a Builder when the driver has no free OUT endpoint, the requested ep_addr is already allocated, or the type/size combination exceeds peripheral capabilities.

Common situations: Composite USB configurations with many OUT endpoints (CDC + MSC + HID + vendor) exceeding hardware endpoint count; explicit address collisions; requesting bulk with packet sizes unsupported by the peripheral.

Related errors

AI-assisted analysis of embassy-rs/embassy@463a07b963 (2026-09-10). Data as JSON: /api/errors/d2e59f34b62956fe. Report an issue: GitHub.

Appendix: source

Thrown at embassy-usb/src/builder.rs:572

        self.endpoint_descriptor(ep.info(), synchronization_type, usage_type, extra_fields);

        ep
    }

    /// Allocate an OUT endpoint, without writing its descriptor.
    ///
    /// Use for granular control over the order of endpoint and descriptor creation.
    pub fn alloc_endpoint_out(
        &mut self,
        ep_type: EndpointType,
        ep_addr: Option<EndpointAddress>,
        max_packet_size: u16,
        interval_ms: u8,
    ) -> D::EndpointOut {
        self.builder
            .driver
            .alloc_endpoint_out(ep_type, ep_addr, max_packet_size, interval_ms)
            .expect("alloc_endpoint_out failed")
    }

    fn endpoint_out(
        &mut self,
        ep_type: EndpointType,
        ep_addr: Option<EndpointAddress>,
        max_packet_size: u16,
        interval_ms: u8,
        synchronization_type: SynchronizationType,
        usage_type: UsageType,
        extra_fields: &[u8],
    ) -> D::EndpointOut {
        let ep = self.alloc_endpoint_out(ep_type, ep_addr, max_packet_size, interval_ms);
        self.endpoint_descriptor(ep.info(), synchronization_type, usage_type, extra_fields);

        ep
    }

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