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Re: [PATCH] arm64: dts: mt8173: add clock_null

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  Re: [PATCH] arm64: dts: mt8173: add clock_null James Liao <jamesjj.liao@mediatek.com> - 2015-06-24 10:00 +0200
    Re: [PATCH] arm64: dts: mt8173: add clock_null James Liao <jamesjj.liao@mediatek.com> - 2015-07-02 04:10 +0200

#1171156 — Re: [PATCH] arm64: dts: mt8173: add clock_null

Hi Heiko,

On Mon, 2015-06-22 at 14:53 +0200, Heiko Stübner wrote:
> Hi James,
> 
> Am Montag, 22. Juni 2015, 11:38:37 schrieb James Liao:
> > On Fri, 2015-06-19 at 13:36 +0200, Heiko Stuebner wrote:
> > 
> > Some clocks such as clkph_mck_o, we don't really care where they come
> > from and what frequencies are. We model these clocks just because they
> > or their derived clocks can be the source of topckgen muxes. Is there a
> > better way to model "don't care" clocks?
> 
> There are two different concepts at work here. You might not care in your 
> kernel driver implementation _at the moment_ where the clocks come from; but 
> the devicetree is supposed to model how the hardware is structured and not 
> contain implementation specific details.

If we model "don't care" clocks inside the driver (i.e. create clk_null
in clock driver), then we don't need to model the dummy clock in device.
Is it an acceptable way?

> So the clock tree should be modeled according to how the hardware is layed out 
> not how you want to use the clocks at the moment :-) .
> 
> It would it any case be good, if you could describe where these clocks come 
> from in the hardware, so we can find the best solution on how to model those.

In fact, I don't know where these clocks come from at all, especially
when they come from outside of SoC. Besides, some clocks don't need to
model in CCF, but they can be the source of clocks that controlled by
CCF.

I don't think ALL clocks on a SoC need to be handled in CCF, so there
should be some clocks don't have a "real" or "correct" parent. In
current implementation I use a dummy clock (clk_null) to be the unreal
parent.

Do you think we should model as more clocks as we can in CCF even we
don't need them? If no, how do we handle those clocks which are not
handled in CCF but can be parent of CCF clocks?


Best regards,

James


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#1175661

Hi Sascha,

On Wed, 2015-07-01 at 08:49 +0200, Sascha Hauer wrote:
> On Tue, Jun 30, 2015 at 05:07:09PM +0800, James Liao wrote:
> > There are 4 clocks which are derived from clk_null directly in current
> > topckgen implementation:
> > 
> > 	clkph_mck_o, dpi_ck, usb_syspll_125m, hdmitx_dig_cts
> > 
> > Our designer mentioned 2 things about external clocks:
> > 
> > 1. These 4 clocks come from analog macro, not from PLL, nor from
> > external clocks directly.
> 
> Ok, this means there actually are clocks. We can't control these clock and
> they have some known or unknown rate. This makes them fixed clocks. Just
> specify them in the device tree and you are done. Give them reasonable
> names and the rate if you know it, 0 otherwise.
> 
> The problem is not that you use fixed clocks for non software
> controllable clocks of unknwown rates, but that you try to use a single
> clock for all of them and name it 'dummy' or 'null'. Name it
> 
> dpi_ck {
> 	compatible = "fixed-clock";
> 	rate = <0>; /* unknown, generated by some Analog block */
> };
> 
> Technically it's the same, but it sounds much more professional and like
> you know what you are doing ;)

These clocks are SoC internal clocks. Is it suitable to specify them in
the device tree?

According to your and Heiko's suggestion, it looks the best way should
be specifying these clocks in the driver code without a dummy parent.
i.e.:

Original code:

FACTOR(CLK_TOP_CLKPH_MCK_O, "clkph_mck_o", "clk_null", 1, 1),
FACTOR(CLK_TOP_DPI, "dpi_ck", "clk_null", 1, 1),
FACTOR(CLK_TOP_USB_SYSPLL_125M, "usb_syspll_125m", "clk_null", 1, 1),
FACTOR(CLK_TOP_HDMITX_DIG_CTS, "hdmitx_dig_cts", "clk_null", 1, 1),

New code:

FIXED(CLK_TOP_CLKPH_MCK_O, "clkph_mck_o", UNKNOWN_RATE),
FIXED(CLK_TOP_DPI, "dpi_ck", UNKNOWN_RATE),
FIXED(CLK_TOP_USB_SYSPLL_125M, "usb_syspll_125m", UNKNOWN_RATE),
FIXED(CLK_TOP_HDMITX_DIG_CTS, "hdmitx_dig_cts", UNKNOWN_RATE),

Then we don't need to specify a dummy clock such as clk_null in device
tree.


Best regards,

James

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