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The i40e device has a different clock rate depending on the current link speed. This requires using a different increment rate for the PTP clock registers. For slower link speeds, the base increment value is larger. Directly multiplying the larger increment value by the parts per billion adjustment might overflow. To avoid this, the i40e implementation defaults to using the lower increment value and then multiplying the adjustment afterwards. This causes a loss of precision for lower link speeds. We can fix this by using mul_u64_u64_div_u64 instead of performing the multiplications using standard C operations. On X86, this will use special instructions that perform the multiplication and division with 128bit intermediate values. For other architectures, the fallback implementation will limit the loss of precision for large values. Small adjustments don't overflow anyways and won't lose precision at all. This allows first multiplying the base increment value and then performing the adjustment calculation, since we no longer fear overflowing. It also makes it easier to convert to the even more precise .adjfine implementation in a following change. 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.