feat: add usb msc support for nrf52840

[mikesmitty]

I don't have an nrf52840 on hand to test yet (should have one tomorrow), but I figured I'd push this up in case @aykevl or anyone else wanted to give it a whirl
#4844 (comment)

[aykevl]

I tried this, but sadly it doesn't work.
I haven't investigated why it doesn't work though.

[mikesmitty]

No worries, my nrf52840 just showed up so I'll investigate

[deadprogram]

@mikesmitty any update on this? I would really like to have it for something... 😸

[mikesmitty]

Oh yeah, this fell off my radar. I've got a debugger just about set up with an nrf52840 now so I can dig into this again

[mikesmitty]

Ok, at least in my testing it looks like it's just a problem with m.buf being too small. My test device has a write block size of 4 bytes, which I didn't think would ever happen. The m.buf buffer is set, perhaps a bit naively, to use the write block size as a capacity, but it needs to be 64 bytes at a minimum to be able to store a full usb packet and it's panicking. #5017 also modifies these buffer sizes but in a more expansive way so this will likely end up just being a temporary patch. I just saw Ayke commented again on that one so I'll swap back to that one next

[deadprogram]

Pinging Dr. @mikesmitty 😸

[mikesmitty]

Sorry, been getting distracted a lot recently haha. I'll start working on this again this weekend

[deadprogram]

Thank you!

[ysoldak]

I gave it a try today on XIAO BLE board.

"usb-storage" example compiles and flashes to the board.

Mac can see the device, but can't really use it. I've tried to "Erase" it with Disk Utility, that just hangs.

See screenshots.

[ysoldak]

Same happens with xiao-rp2040 target though, which as I understood is supposed to work, so either nrf version already works as expected OR we need a better example to try with OR need an instruction how to initialize the storage device.