linux-kernel - Re: [PATCH] arm64: dts: mt8173: add clock

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Date:	Wed, 1 Jul 2015 08:49:35 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	James Liao <jamesjj.liao@...iatek.com>
Cc:	Heiko Stübner <heiko@...ech.de>,
	linux-mediatek@...ts.infradead.org,
	Eddie Huang <eddie.huang@...iatek.com>,
	Matthias Brugger <matthias.bgg@...il.com>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, djkurtz@...omium.org,
	Mike Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>, ted.lin@...iatek.com
Subject: Re: [PATCH] arm64: dts: mt8173: add clock_null

On Tue, Jun 30, 2015 at 05:07:09PM +0800, James Liao wrote:
> Hi Heiko,
> 
> 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 ;)

Sascha

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