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IRQ-CPU mapping is round robined on ARM64 to increase
concurrency and allow multiple interrupts to be serviced
at a time. This reduces the need for FIQ.
Signed-off-by: Michael Zoran <mzoran@crowfest.net>
drivers: irqchip: irq-bcm2835: Concurrency fix
The commit shown in Fixes: aims to improve interrupt throughput by
getting the handlers invoked on different CPU cores. It does so (*) by
using an irq_ack hook to change the interrupt routing.
Unfortunately, the IRQ status bits must be cleared at source, which only
happens once the interrupt handler has run - there is no easy way for
one core to claim one of the IRQs before sending the remainder to the
next core on the list, so waking another core immediately results in a
race with a chance of both cores handling the same IRQ. It is probably
for this reason that the routing change is deferred to irq_ack, but that
doesn't guarantee no clashes - after irq_ack is called, control returns
to bcm2836_chained_handler_irq which proceeds to check for other pending
IRQs at a time when the next core is probably doing the same thing.
Since the whole point of the original commit is to distribute the IRQ
handling, there is no reason to attempt to handle multiple IRQs in one
interrupt callback, so the problem can be solved (or at least made much
harder to reproduce) by changing a "while" into an "if", so that each
invocation only handles one IRQ.
(*) I'm not convinced it's as effective as claimed since irq_ack is
called _after_ the interrupt handler, but the author thought it made a
difference.
See: https://github.com/raspberrypi/linux/issues/5214
https://github.com/raspberrypi/linux/pull/1794
Fixes: fd4c9785bd ("ARM64: Round-Robin dispatch IRQs between CPUs.")
Signed-off-by: Phil Elwell <phil@raspberrypi.com>
irqchip: irq-bcm2836: Avoid prototype warning
Declare bcm2836_arm_irqchip_spin_gpu_irq in irq-bcm2836.h to avoid a
compiler warning about a missing prototype.
Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Linux kernel
There are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. Please read Documentation/admin-guide/README.rst first.
In order to build the documentation, use make htmldocs or
make pdfdocs. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.
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