clk: imx: composite-8m: Add imx8m_divider_determine_rate
Currently, certain clocks are derrived as a divider from their
parent clock. For some clocks, even when CLK_SET_RATE_PARENT
is set, the parent clock is not properly set which can lead
to some relatively inaccurate clock values.
Unlike imx/clk-composite-93 and imx/clk-divider-gate, it
cannot rely on calling a standard determine_rate function,
because the 8m composite clocks have a pre-divider and
post-divider. Because of this, a custom determine_rate
function is necessary to determine the maximum clock
division which is equivalent to pre-divider * the
post-divider.
With this added, the system can attempt to adjust the parent rate
when the proper flags are set which can lead to a more precise clock
value.
On the imx8mplus, no clock changes are present.
On the Mini and Nano, this can help achieve more accurate
lcdif clocks. When trying to get a pixel clock of 31.500MHz
on an imx8m Nano, the clocks divided the 594MHz down, but
left the parent rate untouched which caused a calulation error.
Before:
video_pll
594000000
video_pll_bypass
594000000
video_pll_out
594000000
disp_pixel
31263158
disp_pixel_clk
31263158
Variance = -236842 Hz
After this patch:
video_pll
31500000
video_pll_bypass
31500000
video_pll_out
31500000
disp_pixel
31500000
disp_pixel_clk
31500000
Variance = 0 Hz
All other clocks rates and parent were the same.
Similar results on imx8mm were found.
Fixes:
690dccc4a0bf ("Revert "clk: imx: composite-8m: Add support to determine_rate"")
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Tested-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20230506195325.876871-1-aford173@gmail.com
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>