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Patch series "mul_u64_u64_div_u64: new implementation", v3. This provides an implementation for mul_u64_u64_div_u64() that always produces exact results. This patch (of 2): Library facilities must always return exact results. If the caller may be contented with approximations then it should do the approximation on its own. In this particular case the comment in the code says "the algorithm ... below might lose some precision". Well, if you try it with e.g.: a = 18446462598732840960 b = 18446462598732840960 c = 18446462598732840961 then the produced answer is 0 whereas the exact answer should be 18446462598732840959. This is _some_ precision lost indeed! Let's reimplement this function so it always produces the exact result regardless of its inputs while preserving existing fast paths when possible. Uwe said: : My personal interest is to get the calculations in pwm drivers right. : This function is used in several drivers below drivers/pwm/ . With the : errors in mul_u64_u64_div_u64(), pwm consumers might not get the : settings they request. Although I have to admit that I'm not aware it : breaks real use cases (because typically the periods used are too short : to make the involved multiplications overflow), but I pretty sure am : not aware of all usages and it breaks testing. : : Another justification is commits like : https://git.kernel.org/tip/77baa5bafcbe1b2a15ef9c37232c21279c95481c, : where people start to work around the precision shortcomings of : mul_u64_u64_div_u64(). Link: https://lkml.kernel.org/r/20240707190648.1982714-1-nico@fluxnic.net Link: https://lkml.kernel.org/r/20240707190648.1982714-2-nico@fluxnic.net Signed-off-by: Nicolas Pitre <npitre@baylibre.com> Tested-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com> Reviewed-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com> Tested-by: Biju Das <biju.das.jz@bp.renesas.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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 reStructuredText 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.