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Convert interrupt handler to per-ep-thread-irq
As it turns out some DWC3-commands ~1ms to complete. Currently we spin until the command completes which is bad.
dwc core implements a demultiplexing irq chip for interrupts per endpoint. The interrupt numbers are allocated during probe and belong to the device. If MSI provides per-endpoint interrupt this dummy interrupt chip can be replaced with “real” interrupts.
interrupts are requested / allocated on usb_ep_enable() and removed on usb_ep_disable(). Worst case are 32 interrupts, the lower limit is two for ep0/1.
dwc3_send_gadget_ep_cmd()will sleep in wait_for_completion_timeout() until the command completes.
the interrupt handler is split into the following pieces:
primary handler of the device goes through every event and calls
generic_handle_irq()for event it. On return from
generic_handle_irq()in acknowledges the event counter so interrupt goes away (eventually).
threaded handler of the device none
primary handler of the EP-interrupt reads the event and tries to process it. Everything that requires sleeping is handed over to the Thread. The event is saved in an per-endpoint data-structure. We probably have to pay attention not to process events once we handed something to thread so we don’t process event X prio Y where X > Y.
threaded handler of the EP-interrupt handles the remaining EP work which might sleep such as waiting for command completion.
There should be no increase in latency since the interrupt-thread has a high priority and will be run before an average task in user land (except the user changed priorities).