linux-kernel - Re: [PATCHv2 4/5] clk: samsung: exynos5410: Add fixed rate clocks

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Date:	Thu, 31 Jul 2014 23:01:03 +0200
From:	Humberto Naves <hsnaves@...il.com>
To:	Sylwester Nawrocki <s.nawrocki@...sung.com>
Cc:	linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, Kukjin Kim <kgene.kim@...sung.com>,
	Tomasz Figa <t.figa@...sung.com>,
	Thomas Abraham <ta.omasab@...il.com>,
	Andreas Farber <afaerber@...e.de>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>
Subject: Re: [PATCHv2 4/5] clk: samsung: exynos5410: Add fixed rate clocks

Hi,

On Thu, Jul 31, 2014 at 1:45 PM, Sylwester Nawrocki
<s.nawrocki@...sung.com> wrote:
> Can you explain what is rationale behind this change ? Is it related to
> suspend/resume ordering ?

I had forgotten, but now remember the reason why I did this. If you
see the current implementation of clk-exynos5410, you will notice it
heavily depends on the clock "fin_pll". On the other hand, this clock
exists because in the current dtb (exynos5410-smdk5410.dts), there is
a node fin_pll such as

       fin_pll: xxti {
               compatible = "fixed-clock";
               clock-frequency = <24000000>;
               clock-output-names = "fin_pll";
               #clock-cells = <0>;
       };

So far so good. But the real problem comes in when I check the rate of
fin_pll to determine if I should install the rate table or not (and I
really need this for my patch). More specifically

       if (_get_rate("fin_pll") == 24 * MHZ) {
               exynos5410_plls[apll].rate_table = apll_24mhz_tbl;
               exynos5410_plls[cpll].rate_table = cpll_24mhz_tbl;
               exynos5410_plls[kpll].rate_table = kpll_24mhz_tbl;
               exynos5410_plls[dpll].rate_table = dpll_24mhz_tbl;
               exynos5410_plls[epll].rate_table = epll_24mhz_tbl;
               exynos5410_plls[ipll].rate_table = ipll_24mhz_tbl;
       }

I *have* to determine if the rate of fin_pll is 24MHz, and this is
impossible to do if fin_pll is not available. Moreover, there is no
way I can ensure that the fixed clock provider for fin_pll was
initialized before mine, so there is chance that _get_rate won't work.
The only way I fix that is to set the dependency explicitly in the
dtb, by adding the fin_pll clock as required resource.

              clock: clock-controller@...10000 {
                      compatible = "samsung,exynos5410-clock";
                      reg = <0x10010000 0x30000>;
                      #clock-cells = <1>;
                      /* Add the parent clock */
                      clocks = <&fin_pll>;
                      clock-names = "fin_pll";
              };

But in any case, the bindings with the DTB must be changed one way or
another, because I *really* need to use fin_pll on my driver
registration. If you agree with this alternative solution I previously
described, I can change that in the next version of the patch series.

Best regards,
Humberto

> Obviously it breaks the kernel/dtb compatibility. We should be moving
> in opposite direction, i.e. completely remove the custom samsung fixed
> rate clocks.
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