irqchip: armada-370-xp: fix MSI race condition

In the Armada 370/XP driver, when we receive an IRQ 1, we read the
list of doorbells that caused the interrupt from register
ARMADA_370_XP_IN_DRBEL_CAUSE_OFFS. This gives the list of MSIs that
were generated. However, instead of acknowledging only the MSIs that
were generated, we acknowledge *all* the MSIs, by writing
~MSI_DOORBELL_MASK in the ARMADA_370_XP_IN_DRBEL_CAUSE_OFFS register.

This creates a race condition: if a new MSI that isn't part of the
ones read into the temporary "msimask" variable is fired before we
acknowledge all MSIs, then we will simply loose it.

It is important to mention that this ARMADA_370_XP_IN_DRBEL_CAUSE_OFFS
register has the following behavior: "A CPU write of 0 clears the bits
in this field. A CPU write of 1 has no effect". This is what allows us
to simply write ~msimask to acknoledge the handled MSIs.

Notice that the same problem is present in the IPI implementation, but
it is fixed as a separate patch, so that this IPI fix can be pushed to
older stable versions as appropriate (all the way to 3.8), while the
MSI code only appeared in 3.13.

Signed-off-by: Lior Amsalem <alior@marvell.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>

drivers/irqchip/irq-armada-370-xp.c

@@ -381,7 +381,7 @@ armada_370_xp_handle_irq(struct pt_regs *regs)
 						ARMADA_370_XP_IN_DRBEL_CAUSE_OFFS)
 				& PCI_MSI_DOORBELL_MASK;
 
-			writel(~PCI_MSI_DOORBELL_MASK, per_cpu_int_base +
+			writel(~msimask, per_cpu_int_base +
 			       ARMADA_370_XP_IN_DRBEL_CAUSE_OFFS);
 
 			for (msinr = PCI_MSI_DOORBELL_START;