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embassy-rs / embassy

Modern embedded framework, using Rust and async.
https://embassy.dev
Apache License 2.0
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embassy-usb: Async control request handler #2554

Open 0e4ef622 opened 9 months ago

0e4ef622 commented 9 months ago

Currently, it does not seem possible to add custom async fn USB control request handlers. Is there any way to support this?

Dirbaio commented 9 months ago

it's a bit complicated, control handlers use dyn and async traits can't use dyn. It'd require using actual generics, which i'm not sure how it'd work because there's N handlers, not just 1. Each class can add its own handler. And it'd probably make the type signature of UsbDevice much more complex.

what's your use case for async control handlers?

0e4ef622 commented 9 months ago

My control handler needs to talk to an spi device but at the same time, there's a loop future that runs every 5 ms which reads data from the same spi device.

I wanted to use an async mutex for this but that clearly won't work, so currently I'm just using a refcell and blocking calls in the handler. If it fails to borrow, I return Rejected.

Another solution I thought about was to have another loop and have the control handler communicate via channels to handle the request but that only works for control out not control in since I can't send return values.