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295de9d0d0
The mainpll and univpll clocks are used as clock sources for multiple
peripherals of different kind, some of which are critical (like AXIs);
a rate change on any of these two will produce a rate change on many
devices and that's likely to produce system instability if not done
correctly: this is the reason why we have "fixed factor" clocks, used
by MUX clocks to provide different rates based on PLL output dividers.
Though, there's one fundamental issue that must be resolved somehow:
When performing GPU DVFS, we get a rate request that will try to change
the frequency of MAINPLL due to the CLK_TOP_MFG mux having clk26m,
mfgpll (the GPU dedicated PLL), mainpll_d3, mainpll_d5 (fixed factor
dividers) as possible parents.
In order to solve that, there are two ways:
1. Add new "fake" mainpll_d3_fixed, mainpll_d5_fixed clocks, clones
of mainpll_d3, mainpll_d5 clocks, for the only purpose of not
declaring CLK_SET_RATE_PARENT; or
2. Simply drop said flag from the original dividers.
After some careful validation, I cannot see anything calling a rate
change request during runtime for MAINPLL, nor for UNIVPLL (which would,
again, mean that we're reclocking lots of peripherals at once!), so it
is safe *and sane* to simply remove the CLK_SET_RATE_PARENT flag to all
of the main/univpll fixed factor divider clocks.
Besides, if for any (doubtful) reason main/univpll rate change will be
required in the future, it's still possible to call that on the PLL main
clocks, so we're still covered anyway.
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
Link: https://lore.kernel.org/r/20221024102307.33722-3-angelogioacchino.delregno@collabora.com
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.