[U-Boot] [PATCH v2 2/6] clk: rk3399: fix off-by one during rate calculation in i2c/spi_set_rate

[Simon Glass <sjg@chromium.org>]

On 29 March 2017 at 05:31, Philipp Tomsich
<philipp.tomsich@theobroma-systems.com> wrote:
> For the RK3399, i2c_set_rate (and by extension: our spi_set_rate,
> which had been mindlessly following the template of the i2c_set_rate
> implementation) miscalculates the rate returned due to a off-by-one
> error resulting from the following sequence of events:
>   1. calculates 'src_div := src_freq / target_freq'
>   2. stores 'src_div - 1' into the register (the actual divider applied
>      in hardware is biased by adding 1)
>   3. returns the result of the DIV_RATE(src_freq, src_div) macro, which
>      expects the (decremented) divider from the hardware-register and
>      implictly adds 1 (i.e. 'DIV_RATE(freq, div) := freq / (div + 1)')
>
> This can be observed with the SPI driver, which sets a rate of 99MHz
> based on the GPLL frequency of 594MHz: the hardware generates a clock
> of 99MHz (src_div is 6, the bitfield in the register correctly reads 5),
> but reports a frequency of 84MHz (594 / 7) on return.
>
> To fix, we have two options:
>  * either we bias (i.e. "DIV_RATE(GPLL, src_div - 1)"), which doesn't
>    make for a particularily nice read
>  * we simply call the i2c/spi_get_rate function (introducing additional
>    overhead for the additional register-read), which reads the divider
>    from the register and then passes it through the DIV_RATE macro
>
> Given that this code is not time-critical, the more readable solution
> (i.e. calling the appropriate get_rate function) is implemented in this
> change.
>
> Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
> Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>
>
> ---
>
> Changes in v2:
> - fixes an off-by-one for the RK3399 that cause the SPI module input
>   clock to be misstated as 84MHz (even though it was running at 99MHz)
>
>  drivers/clk/rockchip/clk_rk3399.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Acked-by: Simon Glass <sjg@chromium.org>