Initial thread here: https://www.spinics.net/lists/linux-clk/msg21682.html > 27 ???. 2017 ?., ? 4:06, Stephen Boyd <sboyd at codeaurora.org> ???????(?): > > Are these limits the min/max limits that the parent clk can > output at? Or the min/max limits that software has constrained on > the clk? > Don?t know how to answer. For example, parent can output 768MHz, but some IP work unstable with that parent rate. This issues was observed by me and I didn?t get official confirmation from rockchip. So, I limit such clock to 192MHz using clk_set_max_rate(). May be I have to limit clk rate using another approach. Anybody from rockchip can confirm that? Or may be contact me clarifying details? > I haven't looked in detail at this > rockchip_fractional_approximation() code, but it shouldn't be > doing the work of both the child rate determination and the > parent rate determination in one place. It should work with the > parent to figure out the rate the parent can provide and then > figure out how to achieve the desired rate from there. Here is clock tree: clock rate ----- ---- xin24m 24000000 pll_gpll 768000000 gpll 768000000 i2s_src 768000000 i2s0_pre 192000000 i2s0_frac 16384000 sclk_i2s0 16384000 I limit i2s0_pre rate to 192MHz in order to I2S IP work properly. rockchip_fractional_approximation() get called for i2s0_frac. if i2s0_pre rate is 20x times less than i2s0_frac, than rockchip_fractional_approximation() tries to set i2s0_pre rate to i2s_src rate. It tries to increase it?s parent rate in order to maximise relation between nominator and denominator. If I convert rockchip_fractional_approximation() to rockchip_determine_rate(), than I get min=0, max=192MHz limits inside rockchip_determine_rate(). May be there should be new logic inside clk framework based on some new clk flags, that will provide max=768MHz for rockchip_determine_rate(). Also found, that rockchip_fractional_approximation() increase parents rate unconditionally without taking into account CLK_SET_RATE_PARENT flag. Stephen, thanks a lot for deep description of determine_rate() background. I?ll taking some time thinking about possible solutions. Regards, Alexander.
