linux-kernel - Re: [PATCH v3 2/8] ARM: dts: sun7i: Add GMAC clock node to sun7i DTSI

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Message-ID: <20140204091316.GJ25625@lukather>
Date:	Tue, 4 Feb 2014 10:13:16 +0100
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Chen-Yu Tsai <wens@...e.org>
Cc:	Emilio Lopez <emilio@...pez.com.ar>,
	Mike Turquette <mturquette@...aro.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	linux-sunxi <linux-sunxi@...glegroups.com>,
	devicetree <devicetree@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/8] ARM: dts: sun7i: Add GMAC clock node to sun7i DTSI

On Tue, Feb 04, 2014 at 11:06:24AM +0800, Chen-Yu Tsai wrote:
> On Tue, Feb 4, 2014 at 3:34 AM, Maxime Ripard
> <maxime.ripard@...e-electrons.com> wrote:
> > On Mon, Feb 03, 2014 at 11:32:20AM +0800, Chen-Yu Tsai wrote:
> >> The GMAC uses 1 of 2 sources for its transmit clock, depending on the
> >> PHY interface mode. Add both sources as dummy clocks, and as parents
> >> to the GMAC clock node.
> >>
> >> Signed-off-by: Chen-Yu Tsai <wens@...e.org>
> >> ---
> >>  arch/arm/boot/dts/sun7i-a20.dtsi | 28 ++++++++++++++++++++++++++++
> >>  1 file changed, 28 insertions(+)
> >>
> >> diff --git a/arch/arm/boot/dts/sun7i-a20.dtsi b/arch/arm/boot/dts/sun7i-a20.dtsi
> >> index 1595e9a..fc7f470 100644
> >> --- a/arch/arm/boot/dts/sun7i-a20.dtsi
> >> +++ b/arch/arm/boot/dts/sun7i-a20.dtsi
> >> @@ -314,6 +314,34 @@
> >>               };
> >>
> >>               /*
> >> +              * The following two are dummy clocks, placeholders used
> >> +              * on gmac_tx clock. The actual frequency and availability
> >> +              * depends on the external PHY, operation mode and link
> >> +              * speed.
> >> +              */
> >
> > If it depends on the external PHY, I guess that means it also depends
> > on the board, right? Or is the GMAC supposed to always have that clock
> > running at 25MHz, no matter what PHY is connected to it?
> 
> What I meant in the comment is that we cannot control the actual clock
> rate of the TX clock. We can only select the source, and this is what
> gmac_tx clock does. It is just a clock mux. The 125MHz and 25MHz clock
> rates are used by the clk_set_rate in the stmmac glue layer to do
> auto-reparenting.
> 
> The board dependent factor is what _type_ of PHY it is using, i.e.
> MII, GMII, or RGMII. If it's MII, the PHY should provide the clock.
> If it's RGMII, the internal clock would be used. GMII is a mix of
> both. The actual clock rate depends on the link speed.
> 
> I should rephrase the comment along the lines of:
> 
> The following two are dummy clocks, placeholders used in the gmac_tx
> clock. The gmac driver will choose one parent depending on the PHY
> interface mode, using clk_set_rate auto-reparenting.
> The actual TX clock rate is not controlled by the gmac_tx clock.

Ok, thanks for the clarification.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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