linux-kernel - Re: [PATCH RFC v3 2/2] clk: Add handling of clk parent and rate assigned from DT

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Message-ID: <2134974.m5sRReBBJh@avalon>
Date:	Fri, 28 Mar 2014 17:44:17 +0100
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Sascha Hauer <s.hauer@...gutronix.de>
Cc:	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	linux-arm-kernel@...ts.infradead.org, mturquette@...aro.org,
	mark.rutland@....com, devicetree@...r.kernel.org,
	linux@....linux.org.uk, gregkh@...uxfoundation.org,
	t.figa@...sung.com, sw0312.kim@...sung.com,
	linux-kernel@...r.kernel.org, kyungmin.park@...sung.com,
	robh+dt@...nel.org, galak@...eaurora.org, grant.likely@...aro.org,
	m.szyprowski@...sung.com
Subject: Re: [PATCH RFC v3 2/2] clk: Add handling of clk parent and rate assigned from DT

Hi Sascha,

On Thursday 27 March 2014 15:47:12 Sascha Hauer wrote:
> On Thu, Mar 27, 2014 at 03:08:10PM +0100, Laurent Pinchart wrote:

[snip]

> > That's clearer indeed. Can the parents and rates depend on the board, or
> > on the SoC only ? We might be getting dangerously close to specifying
> > platform configuration instead of describing the hardware. A real example
> > might be nice to support the discussion.
> 
> This patch comes just at the right time. This is what I do with it:
> 
> #define cko1_sel 57
> #define pll4_audio_div 203
> #define pll4_audio 173
> #define ssi3_sel 47
> 
> &clks {
> 	assigned-clocks {
> 		clocks = <&clks cko1_sel>, <&clks ssi3_sel>, <&clks pll4_audio>;
> 		clock-parents = <&clks pll4_audio_div>, <&clks pll4_audio_div>, <0>;
> 		clock-rates = <0>, <0>, <786432000>;
> 	};
> };
> 
> cko1_sel is a clock that can be routed out of the SoC. In my case it is
> connected the sysclk of an external Audio Codec. ssi3_sel drives my SoC
> internal I2S unit which I use in master mode. The above makes sure that
> the I2S unit and the the external codec both get their clock from the
> audio PLL. The audio PLL is configured to a rate of 786432000Hz which
> is an exact multiple of the desired audio clock.

Thank you for the example.

Are the cko1_sel and ssi3_sel used only by the external audio codec and 
internal I2S unit respectively ? If so, it might make sense to move the 
configuration of their parent to the audio codec and I2S unit DT nodes. 
However, grouping the parent configuration and the pll4 rate configuration in 
a single place makes sense as well. Guidelines are probably needed.

I get a slight feeling of uneasiness about this, probably because we're at the 
boundary between hardware description and system configuration. Encoding in DT 
that "for this particular board this particular clock must be configured this 
particular way" sounds fine to me, but we need to make sure it won't turn to 
software-driven rather than hardware-driven use case descriptions.

-- 
Regards,

Laurent Pinchart

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