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Sebastian Reichel's avatar
Sebastian Reichel authored
The clock framework handles clock rates as "unsigned long", so u32 on
32-bit architectures and u64 on 64-bit architectures.

The current code casts the dividend to u64 on 32-bit to avoid a
potential overflow. For example DIV_ROUND_UP(3000000000, 1500000000)
= (3.0G + 1.5G - 1) / 1.5G = = OVERFLOW / 1.5G, which has been
introduced in commit 9556f9da ("clk: divider: handle integer overflow
when dividing large clock rates").

On 64 bit platforms this masks the divisor, so that only the lower
32 bit are used. Thus requesting a frequency >= 4.3GHz results
in incorrect values. For example requesting 4300000000 (4.3 GHz) will
effectively request ca. 5 MHz. Requesting clk_round_rate(clk, ULONG_MAX)
is a bit of a special case, since that still returns correct values as
long as the parent clock is below 8.5 GHz.

Fix this by switching to DIV_ROUND_UP_NO_OVERFLOW, which cannot
overflow. This avoids any requirements on the arguments (except
that divisor should not be 0 obviously).

Signed-off-by: default avatarSebastian Reichel <sebastian.reichel@collabora.com>
9279ce39
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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 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.