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30eaf02149
The function zynqmp_pll_round_rate is used to find a most appropriate
PLL frequency which the hardware can generate according to the desired
frequency. For example, if the desired frequency is 297MHz, considering
the limited range from PS_PLL_VCO_MIN (1.5GHz) to PS_PLL_VCO_MAX (3.0GHz)
of PLL, zynqmp_pll_round_rate should return 1.872GHz (297MHz * 5).
There are two problems with the current code of zynqmp_pll_round_rate:
1) When the rate is below PS_PLL_VCO_MIN, it can't find a correct rate
when the parameter "rate" is an integer multiple of *prate, in other words,
if "f" is zero, zynqmp_pll_round_rate won't return a valid frequency which
is from PS_PLL_VCO_MIN to PS_PLL_VCO_MAX. For example, *prate is 33MHz
and the rate is 660MHz, zynqmp_pll_round_rate will not boost up rate and
just return 660MHz, and this will cause clk_calc_new_rates failure since
zynqmp_pll_round_rate returns an invalid rate out of its boundaries.
2) Even if the rate is higher than PS_PLL_VCO_MIN, there is still a risk
that zynqmp_pll_round_rate returns an invalid rate because the function
DIV_ROUND_CLOSEST makes some loss in the fractional part. If the parent
clock *prate is 33333333Hz and we want to set the PLL rate to 1.5GHz,
this function will return 1499999985Hz by using the formula below:
value = *prate * DIV_ROUND_CLOSEST(rate, *prate)).
This value is also invalid since it's slightly smaller than PS_PLL_VCO_MIN.
because DIV_ROUND_CLOSEST makes some loss in the fractional part.
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Link: https://lore.kernel.org/r/20220826142030.213805-1-quanyang.wang@windriver.com
Reviewed-by: Shubhrajyoti Datta <shubhrajyoti.datta@amd.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.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.