linux-kernel - Re: [EXT] Re: [PATCH v5 3/3] ARM: imx6plus: optionally enable internal routing of clk_enet

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Date:   Sun, 5 Jul 2020 11:34:18 -0400
From:   Sven Van Asbroeck <thesven73@...il.com>
To:     Andy Duan <fugang.duan@....com>
Cc:     Fabio Estevam <festevam@...il.com>,
        Shawn Guo <shawnguo@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        dl-linux-imx <linux-imx@....com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [EXT] Re: [PATCH v5 3/3] ARM: imx6plus: optionally enable
 internal routing of clk_enet_ref

Hi Andy, thank you so much for your time and attention. See below.

On Sun, Jul 5, 2020 at 10:45 AM Andy Duan <fugang.duan@....com> wrote:
>
> Don't consider it complex, GPR5[9] just select the rgmii gtx source from PAD or internal
> Like：
> GPR5[9] is cleared: PAD -> MAC gtx
> GPR5[9] is set: Pll_enet -> MAC gtx
> As you said, register one clock mux for the selection, assign the clock parent by board dts
> file, but now current clock driver doesn't support GPR clock.

Ok, so for imx6q plus only, we create two new clocks (MAC_GTX and PAD)
and a new clock mux, controlled by GPR5[9]:

  enet_ref-o------>ext>---pad------| \
           |                       |M |----mac_gtx
           o-----------------------|_/

Where M = mux controlled by GPR5[9]

clk_mac_gtx -> clk_pad -> clk_enet_ref is the default. when a board
wants internal routing, it can just do:

&fec {
assigned-clocks = <&clks IMX6QDL_CLK_MAC_GTX>;
assigned-clock-parents = <&clks IMX6QDL_CLK_ENET_REF>;
};

But, how do we manage clk_pad? It is routed externally, and can be
connected to:
- enet_ref (typically via GPIO_16)
- an external oscillator
- an external PHY clock

  ext phy---------| \
                  |  |
  enet_ref-o------|M |----pad------| \
           |      |_/              |  |
           |                       |M |----mac_gtx
           |                       |  |
           o-----------------------|_/


How do we tell the clock framework that clk_pad has a mux that can
be connected to _any_ external clock? and also enet_ref?