On Sat, Oct 19, 2019 at 3:01 AM Thomas Gleixner <t...@linutronix.de> wrote: > > On Sat, 19 Oct 2019, Huacai Chen wrote: > > On Fri, Oct 18, 2019 at 11:15 AM Andy Lutomirski <l...@kernel.org> wrote: > > > > > > On Thu, Oct 17, 2019 at 7:57 PM Huacai Chen <che...@lemote.com> wrote: > > > > > > > > In do_hres(), we currently use whether the return value of __arch_get_ > > > > hw_counter() is negtive to indicate fallback, but this is not a good > > > > idea. Because: > > > > > > > > 1, ARM64 returns ULL_MAX but MIPS returns 0 when clock_mode is invalid; > > > > 2, For a 64bit counter, a "negtive" value of counter is actually valid. > > > > > > s/negtive/negative > > > > > > What's the actual bug? Is it that MIPS is returning 0 but the check > > > is < 0? Sounds like MIPS should get fixed. > > My original bug is what Vincenzo said, MIPS has a boot failure if no > > valid clock_mode, and surely MIPS need to fix. However, when I try to > > fix it, I found that clock_getres() has another problem, because > > __cvdso_clock_getres_common() get vd[CS_HRES_COARSE].hrtimer_res, but > > hrtimer_res is set in update_vdso_data() which relies on > > __arch_use_vsyscall(). > > __arch_use_vsyscall() is a pointless exercise TBH. The VDSO data should be > updated unconditionally so all the trivial interfaces like time() and > getres() just work independently of the functions which depend on the > underlying clocksource. > > This functions have a fallback operation already: > > Let __arch_get_hw_counter() return U64_MAX and the syscall fallback is > invoked. >
My thought was that __arch_get_hw_counter() could return last-1 to indicate failure, which would allow the two checks to be folded into one check. Or we could continue to use U64_MAX and rely on the fact that (s64)U64_MAX < 0, not worry about the cycle counter overflowing, and letting cycles < last catch it. (And we should change it to return s64 at some point regardless -- all the math is signed, so the underlying types should be too IMO.)
