[U-Boot] [PATCH v4 4/7] rockchip: spi: rewrite rkspi_set_clk for a more conservative baudrate setting

[Simon Glass]

On 20 April 2017 at 14:05, Philipp Tomsich
<philipp.tomsich at theobroma-systems.com> wrote:
> The baudrate in rkspi was calculated by using an integer division
> (which implicitly discarded any fractional result), then rounding to
> an even number and finally clamping to 0xfffe using a bitwise AND
> operator.  This introduced two issues:
> 1) for very small baudrates (overflowing the 0xfffe range), the
>    bitwise-AND generates rather random-looking (wildly varying)
>    actual output bitrates
> 2) for higher baudrates, the calculation tends to 'err towards a
>    higher baudrate' with the actual error increasing as the dividers
>    become very small. E.g., with a 99MHz input clock, a request
>    for a 20MBit baudrate (99/20 = 4.95), a 24.75 MBit would be use
>    (which amounts to a 23.75% error)... for a 34 MBit request this
>    would be an actual outbout of 49.5 Mbit (i.e. a 45% error).
>
> This change rewrites the divider selection (i.e. baudrate calculation)
> by making sure that
> a) for the normal case: the largest representable baudrate below the
>    requested rate will be chosen;
> b) for the denormal case (i.e. when the divider can no longer be
>    represented), the lowest representable baudrate is chosen.
>
> Even though the denormal case (b) may be of little concern in real
> world applications (even with a 198MHz input clock, this will only
> happen at below approx. 3kHz/3kBit), our board-verification team kept
> complaining.
>
> Signed-off-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> Tested-by: Klaus Goger <klaus.goger at theobroma-systems.com>
> ---
>
> Changes in v4:
> - added in v4 after receiving complaints from the board-verification
>   team
>
> Changes in v3: None
> Changes in v2: None
>
>  drivers/spi/rk_spi.c | 25 ++++++++++++++++++++++---
>  1 file changed, 22 insertions(+), 3 deletions(-)

Applied to u-boot-rockchip/next, thanks!