Re: [PATCH] vulkan: Add VK_EXT_calibrated_timestamps extension (radv and anv) [v4]

From: "Keith Packard" <keithp@keithp.com>
To: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: mesa-dev <mesa-dev@lists.freedesktop.org>,
	ML dri-devel <dri-devel@lists.freedesktop.org>
Subject: Re: [PATCH] vulkan: Add VK_EXT_calibrated_timestamps extension (radv and anv) [v4]
Date: Tue, 16 Oct 2018 12:34:39 -0700	[thread overview]
Message-ID: <87bm7t8z3k.fsf@keithp.com> (raw)
In-Reply-To: <CAP+8YyE1XVe3GRVqr7Vay69iooMGiOD8_nEAWcnFgRYxEMtt8w@mail.gmail.com>

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Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl> writes:

>> +       end = radv_clock_gettime(CLOCK_MONOTONIC_RAW);
>> +
>> +       uint64_t clock_period = end - begin;
>> +       uint64_t device_period = DIV_ROUND_UP(1000000, clock_crystal_freq);
>> +
>> +       *pMaxDeviation = MAX2(clock_period, device_period);
>
> Should this not be a sum? Those deviations can happen independently
> from each other, so worst case both deviations happen in the same
> direction which causes the magnitude to be combined.

This use of MAX2 comes right from one of the issues raised during work
on the extension:

 8) Can the maximum deviation reported ever be zero?

 RESOLVED: Unless the tick of each clock corresponding to the set of
 time domains coincides and all clocks can literally be sampled
 simutaneously, there isn’t really a possibility for the maximum
 deviation to be zero, so by convention the maximum deviation is always
 at least the maximum of the length of the ticks of the set of time
 domains calibrated and thus can never be zero.

I can't wrap my brain around this entirely, but I think that this says
that the deviation reported is supposed to only reflect the fact that we
aren't sampling the clocks at the same time, and so there may be a
'tick' of error for any sampled clock.

If you look at the previous issue in the spec, that actually has the
pseudo code I used in this implementation for computing maxDeviation
which doesn't include anything about the time period of the GPU.

Jason suggested using the GPU period as the minimum value for
maxDeviation at the GPU time period to make sure we never accidentally
returned zero, as that is forbidden by the spec. We might be able to use
1 instead, but it won't matter in practice as the time it takes to
actually sample all of the clocks is far longer than a GPU tick.

-- 
-keith

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