linux-kernel - Re: [PATCH 6/9] clk: pistachio: Propagate rate changes in the MIPS PLL clock sub-tree

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Date:	Fri, 22 May 2015 10:42:45 -0700
From:	Andrew Bresticker <abrestic@...omium.org>
To:	Ezequiel Garcia <ezequiel.garcia@...tec.com>
Cc:	Linux-MIPS <linux-mips@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mike Turquette <mturquette@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	James Hartley <james.hartley@...tec.com>,
	Govindraj Raja <Govindraj.Raja@...tec.com>,
	Damien Horsley <Damien.Horsley@...tec.com>,
	Kevin Cernekee <cernekee@...omium.org>,
	James Hogan <james.hogan@...tec.com>
Subject: Re: [PATCH 6/9] clk: pistachio: Propagate rate changes in the MIPS
 PLL clock sub-tree

On Thu, May 21, 2015 at 4:57 PM, Ezequiel Garcia
<ezequiel.garcia@...tec.com> wrote:
> This commit passes CLK_SET_RATE_PARENT to the "mips_div",
> "mips_internal_div", and "mips_pll_mux" clocks. This flag is needed for the
> "mips" clock to propagate rate changes up to the "mips_pll" root clock.
>
> Signed-off-by: Govindraj Raja <Govindraj.Raja@...tec.com>
> Signed-off-by: Ezequiel Garcia <ezequiel.garcia@...tec.com>

IIRC the clk core will prefer changing a downstream divider over
propagating the rate change up another level.  So, for example, if
MIPS_PLL is initially 400Mhz and we request a MIPS rate of 200Mhz,
we'll change the first intermediate divider to /2 rather than
propagate the rate change up to MIPS_PLL.  Wouldn't it be more
power-efficient to set the MIPS_PLL directly to the requested rate
rather than using external dividers to divide it down?
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