From: Heiko Stuebner <heiko@sntech.de>
To: linux-mediatek@lists.infradead.org
Cc: Eddie Huang <eddie.huang@mediatek.com>,
        Matthias Brugger <matthias.bgg@gmail.com>, devicetree@vger.kernel.org,
        James Liao <jamesjj.liao@mediatek.com>,
        Sascha Hauer <s.hauer@pengutronix.de>, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, djkurtz@chromium.org
Subject: Re: [PATCH] arm64: dts: mt8173: add clock_null
Date: Fri, 19 Jun 2015 13:36:40 +0200
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Am Donnerstag, 18. Juni 2015, 18:15:03 schrieb Heiko Stuebner:
> Am Donnerstag, 18. Juni 2015, 13:29:11 schrieb Eddie Huang:
> > 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.


Heiko
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