linux-kernel - Re: [PATCH 1/2] clk: rename clk_core_get_boundaries() to clk_hw_get

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Date:   Thu, 28 Dec 2017 15:41:20 +0300
From:   Alexander Kochetkov <al.kochet@...il.com>
To:     Stephen Boyd <sboyd@...eaurora.org>
Cc:     linux-clk@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        linux-rockchip@...ts.infradead.org,
        Michael Turquette <mturquette@...libre.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Elaine Zhang <zhangqing@...k-chips.com>
Subject: Re: [PATCH 1/2] clk: rename clk_core_get_boundaries() to clk_hw_get_boundaries() and expose

Initial thread here:
https://www.spinics.net/lists/linux-clk/msg21682.html

> 27 дек. 2017 г., в 4:06, Stephen Boyd <sboyd@...eaurora.org> написал(а):
> 
> 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.