On 17 April 2015 at 16:08, Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx> wrote: > On Thu, Apr 16, 2015 at 03:37:19PM +0100, Tomeu Vizoso wrote: > > [...] > >> >> I don't know what FIQs are. :-) >> > >> > In short, fast IRQs, it is a separate IRQ line handled as a separate >> > exception source with some private (banked) registers that minimize registers >> > saving/restoring. They are not identical to NMI on x86, since >> > their behaviour (handling) may be overriden by platforms and they >> > can be masked. >> > >> >> ->enter_freeze is entered with interrupts disabled on the local CPU. It is >> >> not supposed to re-enable them. That is, while in the ->enter_freeze callback >> >> routine, the CPU must not be interrupted aby anything other than NMI. >> > >> > It boils down to what FIQs handlers are allowed to do with tick frozen >> > and what they are (may be) currently used for. >> > >> > Russell has more insights on this than I do, in particular what FIQs are >> > currently used for on ARM and if we can leave them enabled safely with tick >> > frozen. >> >> But even if it's currently safe to leave them enabled, is there any >> reason for not disabling them? > > Ok, the point here is: either it is safe, and you leave them enabled, > or it is not and we must disable them *before* enter_freeze() is entered. > > Disabling them in the platform enter_freeze() hook does not make sense, > because this means we run with FIQs enabled with tick frozen, either > it is safe or it is not, it can't be both. I have been looking and asking around, and seems like we should actually leave FIQs enabled when the tick is frozen and only disable them within enter_freeze. My understanding is that FIQ handlers are extremely limited in what they can do, with the main use being NMI-like functionality. There are a handful of other FIQ handlers in mainline, but they don't call into any other kernel code and limit themselves to doing some basic I/O. The only reason why they should be disabled in enter_freeze is that the context of the CPU that is going to a lower power state could become corrupted if a FIQ gets fired during a save or restore operation. Regards, Tomeu > I would ask Russell opinion on this, before making any decision. > > Lorenzo > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- To unsubscribe from this list: send the line "unsubscribe linux-tegra" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
