In todays edition of "stuff that I found in my storage" a PS/2 meme
Image transcription:
mov rax, rbx add rax, rcx HELLO IT'S THE KEYBOARD I HAVE AN IMPORTANT MESSAGE E
this post was submitted on 05 Oct 2024
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Actually it's both, there's polling with a timeout interrupt if an incoming urb hasn't been processed quickly.
https://crlab.ece.ucr.edu/usbgps/ohci_uhci.html
This way you can take them in batches at regular intervals.
It makes sense given that even microcontrollers (which are a lot simpler than microprocessors) have dedicated hardware doing the send/receive part of the work which is independent of the processing core and will only fire interrupts if the send or receive buffers reach an empty state.
This way of doing things was already usual before USB for things like I2C, Serial and SPI and whilst the USB protocol is significantly more complex, the same kind of design was also used for it.