On Mon, Mar 25, 2013 at 09:28:10PM +0000, Rob Herring wrote: > On 03/25/2013 12:26 PM, Russell King - ARM Linux wrote: > > On Thu, Mar 21, 2013 at 11:06:47AM +0000, Mark Rutland wrote: > >> On TC2 this series leads to using the vexpress 24MHz clock as the sched clock > >> in preference to the architected timer: > >> > >> Architected local timer running at 24.00MHz (virt). > >> Switching to timer-based delay loop > >> Registered arch_counter_get_cntvct+0x0/0x14 as sched_clock source > >> sched_clock: 32 bits at 24MHz, resolution 41ns, wraps every 178956ms > >> Registered versatile_read_sched_clock+0x0/0x28 as sched_clock source > >> > >> As they both have the same frequency, neither overrides the other, and > >> whichever gets registered last is used as the sched_clock. As accesses > >> to the architected timer are going to have a much lower overhead, this > >> isn't very nice (and it could be better to use it even if it had a lower > >> frequency). > > > > I'll remind people that sched_clock() is supposed to be functional at > > the point in the boot sequence where the call to sched_init() is called. > > That is after setup_arch() and *before* time_init() is called. > > I count integrator-cp, realview, versatile and non-DT VExpress that do > this (not surprisingly) and 25 platforms or timer implementations plus > arm64 that do sched_clock setup in time_init. Before time_init(), sched_clock() currently returns 0 with the architected timers (though I don't particularly like this for arm64). Marc Rutland has patches to make arch_timer_read_counter() a function which always returns the virtual counter. It requires the CNTVOFF register to be set to 0 on AArch32 during boot. But this way sched_clock() on arm64 would always return meaningful values as we have the architected timers. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
