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Dividing by the result of a division looses precision because the result is rounded twice. E.g. with clk_rate = 48000000 and period = 32760033 the following numbers result: rate = pc->clk_rate >> PWM_DUTY_WIDTH = 187500 hz = DIV_ROUND_CLOSEST_ULL(100ULL * NSEC_PER_SEC, period_ns) = 3052 rate = DIV_ROUND_CLOSEST_ULL(100ULL * rate, hz) = 6144 The exact result would be 6142.5061875 and (apart from rounding) this is found by using a single division. As a side effect is also a tad cheaper to calculate. Also using clk_rate >> PWM_DUTY_WIDTH looses precision. Consider for example clk_rate = 47999999 and period = 106667: mul_u64_u64_div_u64(pc->clk_rate >> PWM_DUTY_WIDTH, period_ns, NSEC_PER_SEC) = 19 mul_u64_u64_div_u64(pc->clk_rate, period_ns, NSEC_PER_SEC << PWM_DUTY_WIDTH) = 20 (The exact result is 20.000062083332033.) With this optimizations also switch from round-closest to round-down for the period calculation. Given that the calculations were non-optimal for quite some time now with variations in both directions which nobody reported as a problem, this is the opportunity to align the driver's behavior to the requirements of new drivers. This has several upsides: - Implementation is easier as there are no round-nearest variants of mul_u64_u64_div_u64(). - Requests for too small periods are now consistently refused. This was kind of arbitrary before, where period_ns < min_period_ns was refused, but in some cases min_period_ns isn't actually implementable and then values between min_period_ns and the actual minimum were rounded up to the actual minimum. Note that the duty_cycle calculation isn't using the usual round-down approach yet. Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Signed-off-by: Thierry Reding <thierry.reding@gmail.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.