linux-kernel - Re: [PATCH v4 2/2] arm64: dts: Add Mediatek SoC MT2712 and evaluation board dts and Makefile

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Date:   Fri, 21 Jul 2017 14:32:26 +0800
From:   YT Shen <yt.shen@...iatek.com>
To:     Matthias Brugger <matthias.bgg@...il.com>
CC:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Marc Zyngier <marc.zyngier@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mars Cheng <mars.cheng@...iatek.com>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-serial@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>, <srv_heupstream@...iatek.com>
Subject: Re: [PATCH v4 2/2] arm64: dts: Add Mediatek SoC MT2712 and
 evaluation board dts and Makefile

On Wed, 2017-07-19 at 11:26 +0200, Matthias Brugger wrote:
> 
> On 07/19/2017 08:48 AM, YT Shen wrote:
> > On Tue, 2017-07-18 at 18:29 +0200, Matthias Brugger wrote:
> >>
> >> On 06/22/2017 11:32 AM, YT Shen wrote:
> >>> This adds basic chip support for Mediatek 2712
> 
> [...]
> 
> >>> +
> >>> +	uart_clk: dummy26m {
> >>> +		compatible = "fixed-clock";
> >>> +		clock-frequency = <26000000>;
> >>> +		#clock-cells = <0>;
> >>> +	};
> >>> +
> 
> [...]
> 
> >>> +
> >>> +	soc {
> >>> +		#address-cells = <2>;
> >>> +		#size-cells = <2>;
> >>> +		compatible = "simple-bus";
> >>> +		ranges;
> >>> +
> >>> +		uart5: serial@...0f000 {
> >>> +			compatible = "mediatek,mt2712-uart",
> >>> +				     "mediatek,mt6577-uart";
> >>> +			reg = <0 0x1000f000 0 0x400>;
> >>> +			interrupts = <GIC_SPI 127 IRQ_TYPE_LEVEL_LOW>;
> >>> +			clocks = <&uart_clk>, <&uart_clk>;
> >>> +			clock-names = "baud", "bus";
> >>> +			status = "disabled";
> >>> +		};
> >>
> >> So baud and bus clock are both 26 MHz?
> > We didn't have CCF clock support in this series.
> > After we have clock source support, we could use the correct clocks to
> > the UARTs and drop the 26MHz fixed rate UART clock.
> > 
> > The bus clock is 26MHz.  The baud clock could be from another clock
> > source, using the same 26MHz fixed clock works also.
> > 
> > 
> > [1] https://patchwork.kernel.org/patch/9670877/
> > [2] https://patchwork.kernel.org/patch/6436021/
> > 
> 
> Yes, just using one 26 MHz clock works, but it uses an deprecated 
> binding, so we should not do this, as through copying from the source of 
> other SoCs we will keep it alive forever. Anyway that's not your case, 
> as you defined the two clocks.
> 
> The device tree should reflect the HW, that's why I asked for the clock 
> frequency of both clocks. I searched the git history and it was never 
> done right before. So you could be the first :)
> 
> Thanks,
> Matthias
Ok, I want to make it clear.  The following example

        baud_clk: dummy26m {
                compatible = "fixed-clock";
                clock-frequency = <26000000>;
                #clock-cells = <0>;
        };

        sys_clk: dummyclk {
                compatible = "fixed-clock";
                clock-frequency = <26000000>;
                #clock-cells = <0>;
        };

	uart0: serial@...02000 {
[...]
                clocks = <&baud_clk>, <&sys_clk>;
[...]
	}

Do you think it is clear to reflect the HW that the baud clock and sys
clock can be different source or we need to choose another frequency?
Thanks.

yt.shen