Messages in this thread | | | Date | Thu, 17 Oct 2019 13:15:15 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [RFC PATCH 1/5] x86: tsc: add tsc to art helpers |
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Hi,
On Tue, 1 Oct 2019, Felipe Balbi wrote: > (sorry for the long delay, got caught up in other tasks)
Delayed by vacation :)
> Thomas Gleixner <tglx@linutronix.de> writes: > > On Thu, 15 Aug 2019, Felipe Balbi wrote: > >> Thomas Gleixner <tglx@linutronix.de> writes: > >> > On Tue, 16 Jul 2019, Felipe Balbi wrote: > >> > > >> > So some information what those interfaces are used for and why they are > >> > needed would be really helpful. > >> > >> Okay, I have some more details about this. The TGPIO device itself uses > >> ART since TSC is not directly available to anything other than the > >> CPU. The 'problem' here is that reading ART incurs extra latency which > >> we would like to avoid. Therefore, we use TSC and scale it to > >> nanoseconds which, would be the same as ART to ns. > > > > Fine. But that's not really correct: > > > > TSC = art_to_tsc_offset + ART * scale; > > From silicon folks I got the equation: > > ART = ECX * EBX / EAX;
What is the content of ECX/EBX/EAX and where is it coming from?
> If I'm reading this correctly, that's basically what > native_calibrate_tsc() does (together with some error checking the safe > defaults). Couldn't we, instead, just have a single function like below? > > u64 convert_tsc_to_art_ns() > { > return x86_platform.calibrate_tsc(); > }
Huch? How is that supposed to work? calibrate_tsc() returns the TSC frequency.
> Another way would be extract the important parts from > native_calibrate_tsc() into a separate helper. This would safe another > call to cpuid(0x15,...);
What for?
The relation between TSC and ART is already established via detect_art() which reads all relevant data out of CPUID(ART_CPUID_LEAF).
We use exactly that information for convert_art_to_tsc() so the obvious solution for calculating ART from TSC is to do the reverse operation.
convert_art_to_tsc() { rem = do_div(art, art_to_tsc_denominator);
res = art * art_to_tsc_numerator; tmp = rem * art_to_tsc_numerator;
do_div(tmp, art_to_tsc_denominator); res += tmp + art_to_tsc_offset; }
which is translated into math:
TSC = ART * SCALE + OFFSET
where
SCALE = N / D
and
N = CPUID(ART_CPUID_LEAF).EAX D = CPUID(ART_CPUID_LEAF).EBX
So the obvious reverse operation is:
ART = (TSC - OFFSET) / SCALE;
Translating that into code should not be rocket science.
Thanks,
tglx
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