[PATCH] arm64: dts: mt8173: add clock_null

Hi Heiko,

On Fri, 2015-06-19 at 13:36 +0200, Heiko Stuebner wrote:

Am Donnerstag, 18. Juni 2015, 18:15:03 schrieb Heiko Stuebner:

quoted

Am Donnerstag, 18. Juni 2015, 13:29:11 schrieb Eddie Huang:

quoted

Add clk_null, which represents clocks that can not / need not
controlled by software.
There are many clocks' parent set to clk_null.

Devicetree is supposed to describe hardware, and ideally not what software
does with it. If the clock simply cannot be controlled by software, it will
still have a rate and I think it should probably be modelled - similarly we
sometimes have fixed regulators that also are not software controllable.


While it might be ok to define dummy clocks as a temporary stopgap, these
should definitly be marked as such. This clk_null at least sounds like there
is no plan to replace this with a real solution at some point.

And of course a bit of context would be cool, to know which type of clocks
this actually replaces.

After looking a bit more into this, I'm feel that the clk_null approach is 
wrong.

For one, even if the clk_null stuff would be ok, the binding doc of the clock 
controller does not describe the need of this specific clock at all.


The other more important point, looking at clk-mt8173 I see at least these 
clocks being set as children of "clk_null":

static const struct mtk_fixed_factor root_clk_alias[] __initconst = {
	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),
};


These look more like they are fed from some external source to the clock 
controller? I did ask Matthias but he also couldn't find anything describing 
where these clocks actually come from.

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?


Best regards,

James