dfu: support "detach"

[karlp]

When in DFU mode, the only way to exit was via a download operation.
"dfu-util -e" to request a detach was simply not supported, so the
bootloader simply remains sitting there
Handle detach via reboot, the usb layer will re-enumerate if it can,
and will boot app/bl based on it's own decisions.

Signed-off-by: Karl Palsson karlp@tweak.net.au

--
this might be me not having used dfu a lot? Is there something I missed?

[devanlai]

Hmm, I guess I never really gave any thought to whether the bootloader should accept a detach command in DFU mode. Appendix A of the spec doesn't show the DFU detach request as a legal request in the dfuIDLE state, so it would fall in the disallowed requests that enter the error state, but it's also not obvious if appendix A is non-normative or not.

I'll have to do some experiments and think about it some more.
Aside from whether or not to allow the detach request, I think the reset could be deferred until after the USB control request is acknowledged using the completion callback, but it's been a while since I looked at the USB stack API.

[karlp]

yeah, I know it's meant to be used for going from app -> dfu, but I couldnt' see any other way of exiting dfu back to app.

[devanlai]

After doing some testing, it seems innocuous enough to allow the detach command - dfu-util is happy with it anyways. I did end up deferring the reset until after sending the response, otherwise dfu-util thinks the detach request failed.