linux-kernel - Re: [EXT] Re: [PATCH v4 2/2] ARM: imx6plus: enable internal routing of clk_enet

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Date:   Wed, 1 Jul 2020 09:51:33 -0400
From:   Sven Van Asbroeck <thesven73@...il.com>
To:     Fabio Estevam <festevam@...il.com>
Cc:     Andy Duan <fugang.duan@....com>, Shawn Guo <shawnguo@...nel.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 v4 2/2] ARM: imx6plus: enable internal routing
 of clk_enet_ref where possible

Andy, Fabio,

Does the following describe the mainline situation?
Please correct if not.

1. imx6 ethernet ref_clk can be generated internally (by imx6) or
   externally (by PHY or oscillator on PCB)
2. if internal, fec's "ptp" clock in devicetree should be
   <&clks IMX6QDL_CLK_ENET_REF>
3. if external, fec's "ptp" clock should be that external clock,
   see 810c0ca879098 ("ARM: imx6q: support ptp and rmii clock from pad")
4. sabresd devicetree describes "ptp" clock as IMX6QDL_CLK_ENET_REF,
   although it's externally supplied (by PHY). This is incorrect.
5. nevertheless sabresd will work, because FEC driver can still work
   when the PTP clock in devicetree is different from supplied PTP clock
6. sabresd plus believes FEC is clocked internally, so flips the bit
   which routes the ptp clock internally
7. this breaks sabresd plus, as default internal clock is unsuitable
8. sabresd is sample board, so all boards based on sabresd may have
   the same issue, and break

Solution 1:
- describe sabresd ptp clock correctly in devicetree
- "clean/correct" solution
- may break other imx6q plus boards in mainline
- may break out-of-tree (private) imx6q plus devicetrees based on
  sabresd

Solution 2:
- on plus, never route PTP clock internally by default
  use a devicetree property instead
- complex solution, hard to understand if newcomer
- prevents sabresd / clones breakage

Solution 3:
- set sabresd ptp clock freq to same as externally supplied clock
- may still break designs based on sabresd
- hard to understand what's actually happening

Other solutions??