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The PTP frequency adjustment code needs to determine an appropriate adjustment given an input scaled_ppm adjustment. We calculate the adjustment to the register by multiplying the base (nominal) increment value by the scaled_ppm and then dividing by the scaled one million value. For very large adjustments, this might overflow. To avoid this, both the scaled_ppm and divisor values are downshifted. We can avoid that on X86 architectures by using mul_u64_u64_div_u64. This helper function will perform the multiplication and division with 128bit intermediate values. We know that scaled_ppm is never larger than the divisor so this operation will never result in an overflow. This improves the accuracy of the calculations for large adjustment values on X86. It is likely an improvement on other architectures as well because the default implementation of mul_u64_u64_div_u64 is smarter than the original approach taken in the ice code. Additionally, this implementation is easier to read, using fewer local variables and lines of code to implement. Signed-off-by: Jacob Keller <jacob.e.keller@intel.com> Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel) Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
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.