Re: [PATCH v2 1/4] clock: Introduce clock_ticks_to_ns()

[Richard Henderson]

On 12/15/20 9:09 AM, Peter Maydell wrote:
> The clock_get_ns() API claims to return the period of a clock in
> nanoseconds. Unfortunately since it returns an integer and a
> clock's period is represented in units of 2^-32 nanoseconds,
> the result is often an approximation, and calculating a clock
> expiry deadline by multiplying clock_get_ns() by a number-of-ticks
> is unacceptably inaccurate.
> 
> Introduce a new API clock_ticks_to_ns() which returns the number
> of nanoseconds it takes the clock to make a given number of ticks.
> This function can do the complete calculation internally and
> will thus give a more accurate result.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> The 64x64->128 multiply is a bit painful for 32-bit and I
> guess in theory since we know we only want bits [95:32]
> of the result we could special-case it, but TBH I don't
> think 32-bit hosts merit much optimization effort these days.
> 
> Changes in v2: saturate the result to INT64_MAX.
> ---
>  docs/devel/clocks.rst | 29 +++++++++++++++++++++++++++++
>  include/hw/clock.h    | 41 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 70 insertions(+)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~