Moving the irq_request/release_resources() callbacks out of the spinlocked,
irq disabled and bus locked region, unearthed an interesting abuse of the
irq_bus_lock/irq_bus_sync_unlock() callbacks.

The OMAP GPIO driver does merily power management inside of them. The
irq_request_resources() callback of this GPIO irqchip calls a function
which reads a GPIO register. That read aborts now because the clock of the
GPIO block is not magically enabled via the irq_bus_lock() callback.

Move the callbacks under the bus lock again to prevent this. In the
free_irq() path this requires to drop the bus_lock before calling
synchronize_irq() and reaquiring it before calling the
irq_release_resources() callback.

The bus lock can't be held because:

   1) The data which has been changed between bus_lock/un_lock is cached in
      the irq chip driver private data and needs to go out to the irq chip
      via the slow bus (usually SPI or I2C) before calling
      synchronize_irq().

      That's the reason why this bus_lock/unlock magic exists in the first
      place, as you cannot do SPI/I2C transactions while holding desc->lock
      with interrupts disabled.

   2) synchronize_irq() will actually deadlock, if there is a handler on
      flight. These chips use threaded handlers for obvious reasons, as
      they allow to do SPI/I2C communication. When the threaded handler
      returns then bus_lock needs to be taken in irq_finalize_oneshot() as
      we need to talk to the actual irq chip once more. After that the
      threaded handler is marked done, which makes synchronize_irq() return.

      So if we hold bus_lock accross the synchronize_irq() call, the
      handler cannot mark itself done because it blocks on the bus
      lock. That in turn makes synchronize_irq() wait forever on the
      threaded handler to complete....

Add the missing unlock of desc->request_mutex in the error path of
__free_irq() and add a bunch of comments to explain the locking and
protection rules.

Fixes: 46e48e25 ("genirq: Move irq resource handling out of spinlocked region")
Reported-and-tested-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Reported-and-tested-by: Tony Lindgren <tony@atomide.com>
Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Not-longer-ranted-at-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>