Chapter 3. Handling IRQs

Additionally to the MUSB controller hardware basic setup and registration, the glue layer is also responsible for handling the IRQs:

static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
	unsigned long   flags;
	irqreturn_t     retval = IRQ_NONE;
	struct musb     *musb = __hci;

	spin_lock_irqsave(&musb->lock, flags);

	musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
	musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
	musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);

	 * The controller is gadget only, the state of the host mode IRQ bits is
	 * undefined. Mask them to make sure that the musb driver core will
	 * never see them set

	if (musb->int_usb || musb->int_tx || musb->int_rx)
		retval = musb_interrupt(musb);

	spin_unlock_irqrestore(&musb->lock, flags);

	return retval;

Here the glue layer mostly has to read the relevant hardware registers and pass their values on to the controller driver which will handle the actual event that triggered the IRQ.

The interrupt handler critical section is protected by the spin_lock_irqsave() and counterpart spin_unlock_irqrestore() functions (line 7 and 24 respectively), which prevent the interrupt handler code to be run by two different threads at the same time.

Then the relevant interrupt registers are read (line 9 to 11):

Note that musb_readb() is used to read 8-bit registers at most, while musb_readw() allows us to read at most 16-bit registers. There are other functions that can be used depending on the size of your device registers. See musb_io.h for more information.

Instruction on line 18 is another quirk specific to the JZ4740 USB device controller, which will be discussed later in Chapter 5.

The glue layer still needs to register the IRQ handler though. Remember the instruction on line 14 of the init function:

static int jz4740_musb_init(struct musb *musb)
	musb->isr = jz4740_musb_interrupt;

	return 0;

This instruction sets a pointer to the glue layer IRQ handler function, in order for the controller hardware to call the handler back when an IRQ comes from the controller hardware. The interrupt handler is now implemented and registered.