Messages in this thread | | | Subject | Re: [PATCH v9 4/9] clocksource/drivers/arm_arch_timer: use readq to get 64-bit CNTVCT | From | Timur Tabi <> | Date | Tue, 26 Jul 2016 09:11:49 -0500 |
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Will Deacon wrote: > The kernel really needs to support both of those platforms :/ > > For the memory-mapped counter registers, the architecture says: > > `If the implementation supports 64-bit atomic accesses, then the > CNTV_CVAL register must be accessible as an atomic 64-bit value.' > > which is borderline tautological. If we take the generous reading that > this means AArch64 CPUs can use readq (and I'm not completely > comfortable with that assertion, particularly as you say that it breaks > the model), then you still need to use readq_relaxed here to avoid a > DSB. Furthermore, what are you going to do for AArch32? readq doesn't > exist over there, and if you use the generic implementation then it's > not atomic. In which case, we end up with the current code, as well as a > readq_relaxed guarded by a questionable #ifdef that is known to break a > supported platform for an unknown performance improvement. Hardly a big > win.
I know Fu dropped this patch, and I don't want to kick a dead horse, but I was wondering if it would be okay to do this:
static u64 arch_counter_get_cntvct_mem(void) { #ifdef readq_relaxed return readq_relaxed(arch_counter_base + CNTVCT_LO); #else u32 vct_lo, vct_hi, tmp_hi;
do { vct_hi = readl_relaxed(arch_counter_base + CNTVCT_HI); vct_lo = readl_relaxed(arch_counter_base + CNTVCT_LO); tmp_hi = readl_relaxed(arch_counter_base + CNTVCT_HI); } while (vct_hi != tmp_hi);
return ((u64) vct_hi << 32) | vct_lo; #endif }
readq and readq_relaxed are defined in arch/arm64/include/asm/io.h. Why would the function exist if AArch64 CPUs can't use it?
Do we need something like ARCH_HAS_64BIT_ATOMIC_READ in order to decide whether readq is safe?
-- Sent by an employee of the Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation.
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