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5ccb9f4632
Our actual timestamp register from the hardware is 36-bits these days.
Depending on the clock base, a few of the low bits may be discarded.
We scale by 52.08 or so (see intel_device_info_timebase_scale()) in
order to convert clock ticks to nanoseconds. This value could take
~43-44 bits to represent. When our timestamp register overflows, the
reported value would overflow at some value which isn't a power-of-two.
For some reason, when we implemented ARB_timer_query for i965 in 2012,
we thought applications would use GL_QUERY_COUNTER_BITS to detect and
handle overflow, expecting our timer queries to overflow at a power-of-
two value. We tried to hack around this by reporting a 36-bit counter,
for what was then a 32-bit(?) timestamp register, with a timebase scale
of 80(?)...and reported the nanoseconds modulo 2^36, with some
handwaving about wrap around times being close enough.
I don't think this is what anyone wants. All other Gallium drivers
(except maybe zink) report 64 here, as does the Intel Windows driver.
The ARB_timer_query spec defines it as:
"If <pname> is QUERY_COUNTER_BITS, the implementation-dependent
number of bits used to hold the query result for <target> will be
placed in <params>. The number of query counter bits may be zero,
in which case the counter contains no useful information."
and it also mentions about overflow:
"If the elapsed time overflows the number of bits, <n>, available to
hold elapsed time, its value becomes undefined. It is recommended,
but not required, that implementations handle this overflow case by
saturating at 2^n - 1."
There's nothing about roll-over happening at power-of-two times, just
that the value returned has to fit in that many bits, and if the value
were to exceed that, it's undefined, optionally saturated to the maximum
representable value.
This patch makes us report 64 like other drivers, and stop taking the
modulus. Technically, our roll-over will happen before we reach the
number of query counter bits, which may or may not be valid. But this
does let us report longer times, and should be more desirable behavior.
Tested-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26058>
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`Mesa <https://mesa3d.org>`_ - The 3D Graphics Library ====================================================== Source ------ This repository lives at https://gitlab.freedesktop.org/mesa/mesa. Other repositories are likely forks, and code found there is not supported. Build & install --------------- You can find more information in our documentation (`docs/install.rst <https://mesa3d.org/install.html>`_), but the recommended way is to use Meson (`docs/meson.rst <https://mesa3d.org/meson.html>`_): .. code-block:: sh $ mkdir build $ cd build $ meson .. $ sudo ninja install Support ------- Many Mesa devs hang on IRC; if you're not sure which channel is appropriate, you should ask your question on `OFTC's #dri-devel <irc://irc.oftc.net/dri-devel>`_, someone will redirect you if necessary. Remember that not everyone is in the same timezone as you, so it might take a while before someone qualified sees your question. To figure out who you're talking to, or which nick to ping for your question, check out `Who's Who on IRC <https://dri.freedesktop.org/wiki/WhosWho/>`_. The next best option is to ask your question in an email to the mailing lists: `mesa-dev\@lists.freedesktop.org <https://lists.freedesktop.org/mailman/listinfo/mesa-dev>`_ Bug reports ----------- If you think something isn't working properly, please file a bug report (`docs/bugs.rst <https://mesa3d.org/bugs.html>`_). Contributing ------------ Contributions are welcome, and step-by-step instructions can be found in our documentation (`docs/submittingpatches.rst <https://mesa3d.org/submittingpatches.html>`_). Note that Mesa uses gitlab for patches submission, review and discussions.