From mboxrd@z Thu Jan 1 00:00:00 1970 From: linux@arm.linux.org.uk (Russell King - ARM Linux) Date: Fri, 20 Jan 2012 17:44:57 +0000 Subject: [PATCH] ARM: ux500: Initialize irq affinity In-Reply-To: References: <1327064356-3603-1-git-send-email-per.xx.fransson@stericsson.com> <20120120130310.GO1068@n2100.arm.linux.org.uk> Message-ID: <20120120174456.GB11845@n2100.arm.linux.org.uk> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Fri, Jan 20, 2012 at 04:45:40PM +0100, Linus Walleij wrote: > On Fri, Jan 20, 2012 at 2:03 PM, Russell King - ARM Linux > wrote: > > On Fri, Jan 20, 2012 at 01:59:16PM +0100, Per Fransson wrote: > >> Use irq_set_affinity() to initialize the kernel view of irq affinity > >> when programming the GIC registers. > > > > Why is this necessary? > > Necessary: we don't quite know. (Maybe should have been RFC, but > this patch is better than being silent of it...) I'm afraid Per Fransson took the thread off the mailing lists, so my reply to him also wasn't public. Here's what I said: | The affinity starts off as "all CPUs", and the implementation is free | to route the IRQ to any of those CPUs. On x86, some hardware is smart | enough to route the IRQ to one of a set of CPUs, choosing which CPU | to target in hardware. | | We don't have that ability, and trying to do that in software proved to | be very problematic to get it to work satisfactorily. Given that there | is a userspace daemon which helps to solve the problem, it was decided | that we wouldn't bother with this i the kernel. | | So, we have to choose a CPU to route the IRQ to from the mask being | requested - and the requested mask may contain anything from a single CPU | to multiple CPUs. | | We choose to route it to the first online CPU in the set. That fits with | the model, and does not require us to report back which CPU it's currently | routed to. Indeed, architectures such as x86 can't tell you which CPU | the next interrupt will hit. | | So we shouldn't even try to do this on ARM. Yes, we end up with all IRQs | targetting CPU0 at boot, and with the masks showing 'all CPUs' but that's | not a problem. Really not a problem.