@@ -2,8 +2,66 @@
* Copyright (C) 1992, 1998-2004 Linus Torvalds, Ingo Molnar
+ * Copyright (C) 2010 SUSE Linux Products GmbH
+ * Copyright (C) 2010 Tejun Heo <firstname.lastname@example.org>
- * This file contains spurious interrupt handling.
+ * There are two ways interrupt handling can go wrong - too few or too
+ * many. Due to misrouting or other issues, sometimes IRQs don't
+ * reach the driver while at other times an interrupt line gets stuck
+ * and a continuous spurious interrupts are generated.
+ * This file implements workaround for both cases. Lost interrupts
+ * are handled by IRQ expecting and watching, and spurious interrupts
+ * by spurious polling. All mechanisms need IRQF_SHARED to be set on
+ * the irqaction in question.
+ * Both lost interrupt workarounds require cooperation from drivers
+ * and can be chosen depending on how much information the driver can
+ * provide.
+ * - IRQ expecting
+ * IRQ expecting is useful when the driver can tell when IRQs can be
+ * expected; in other words, when IRQs are used to signal completion
+ * of host initiated operations. This is the surest way to work
+ * around lost interrupts.
+ * When the controller is expected to raise an IRQ, the driver
+ * should call expect_irq() and, when the expected event happens or
+ * times out, unexpect_irq(). IRQ subsystem polls the interrupt
+ * inbetween.
+ * As interrupts tend to keep working if it works at the beginning,
+ * IRQ expecting implements "verified state". After certain number
+ * of successful IRQ deliveries, the irqaction becomes verified and
+ * much longer polling interval is used.
+ * - IRQ watching
+ * This can be used when the driver doesn't know when to exactly
+ * expect and unexpect IRQs. Once watch_irq() is called, the
+ * irqaction is slowly polled for certain amount of time (1min). If
+ * IRQs are missed during that time, the irqaction is marked and
+ * actively polled; otherwise, the watching is stopped.
+ * In the most basic case, drivers can call this right after
+ * registering an irqaction to verify IRQ delivery. In many cases,
+ * if IRQ works at the beginning, it keeps working, so just calling
+ * watch_irq() once can provide decent protection against misrouted
+ * IRQs. It would also be a good idea to call watch_irq() when
+ * timeouts are detected.
+ * - Spurious IRQ handling
+ * All IRQs are continuously monitored and spurious IRQ handling
+ * kicks in if there are too many spurious IRQs. The IRQ is
+ * disabled and the registered irqactions are polled. The IRQ is
+ * given another shot after certain number IRQs are handled or an
+ * irqaction is added or removed.
+ * All of the above three mechanisms can be used together. Spurious
+ * IRQ handling is enabled by default and drivers are free to expect
+ * and watch IRQs as they see fit.
@@ -17,6 +75,20 @@
+ * I spent quite some time thinking about each parameter but they
+ * still are just numbers pulled out of my ass. If you think your ass
+ * is prettier than mine, please go ahead and suggest better ones.
+ * Most parameters are intentionally fixed constants and not
+ * adjustable through API. The nature of IRQ delivery failures isn't
+ * usually dependent on specific drivers and the timing parameters are
+ * more about human perceivable latencies rather than any specific
+ * controller timing details, so figuring out constant values which
+ * can work for most cases shouldn't be too hard. This allows tighter
+ * control over polling behaviors, eases future changes and makes the
+ * interface easy for drivers.
/* irqfixup levels */
IRQFIXUP_SPURIOUS = 0, /* spurious storm detection */