Update Linux to v5.4.2
Change-Id: Idf6911045d9d382da2cfe01b1edff026404ac8fd
diff --git a/Documentation/usb/dwc3.rst b/Documentation/usb/dwc3.rst
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+===========
+DWC3 driver
+===========
+
+
+TODO
+~~~~
+
+Please pick something while reading :)
+
+- 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.
+
+ Implementation idea:
+
+ - 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.
+
+ Latency:
+
+ 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).