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From:   Shenwei Wang <shenwei.wang@nxp.com>
To:     Will Deacon <will@kernel.org>,
        "Russell King (Oracle)" <linux@armlinux.org.uk>
CC:     Andrew Halaney <ahalaney@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        Shawn Guo <shawnguo@kernel.org>,
        Sascha Hauer <s.hauer@pengutronix.de>,
        Neil Armstrong <neil.armstrong@linaro.org>,
        Kevin Hilman <khilman@baylibre.com>,
        Vinod Koul <vkoul@kernel.org>, Chen-Yu Tsai <wens@csie.org>,
        Jernej Skrabec <jernej.skrabec@gmail.com>,
        Samuel Holland <samuel@sholland.org>,
        Giuseppe Cavallaro <peppe.cavallaro@st.com>,
        Alexandre Torgue <alexandre.torgue@foss.st.com>,
        Jose Abreu <joabreu@synopsys.com>,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        Fabio Estevam <festevam@gmail.com>,
        dl-linux-imx <linux-imx@nxp.com>,
        Jerome Brunet <jbrunet@baylibre.com>,
        Martin Blumenstingl <martin.blumenstingl@googlemail.com>,
        Bhupesh Sharma <bhupesh.sharma@linaro.org>,
        Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>,
        Simon Horman <simon.horman@corigine.com>,
        Bartosz Golaszewski <bartosz.golaszewski@linaro.org>,
        Wong Vee Khee <veekhee@apple.com>,
        Revanth Kumar Uppala <ruppala@nvidia.com>,
        Jochen Henneberg <jh@henneberg-systemdesign.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
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        <linux-stm32@st-md-mailman.stormreply.com>,
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        <linux-amlogic@lists.infradead.org>,
        "imx@lists.linux.dev" <imx@lists.linux.dev>,
        Frank Li <frank.li@nxp.com>
Subject: RE: [EXT] Re: [PATCH v2 net 2/2] net: stmmac: dwmac-imx: pause the
 TXC clock in fixed-link
Thread-Topic: [EXT] Re: [PATCH v2 net 2/2] net: stmmac: dwmac-imx: pause the
 TXC clock in fixed-link
Thread-Index: AQHZwJ6ZmPtrsmRv9UGenRTta+C4Xq/N8VmAgAFcAoCAAAPhAIAAB8xg
Date:   Fri, 28 Jul 2023 16:09:29 +0000
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> -----Original Message-----
> From: Will Deacon <will@kernel.org>
> Sent: Friday, July 28, 2023 10:36 AM
> To: Russell King (Oracle) <linux@armlinux.org.uk>
> Cc: Andrew Halaney <ahalaney@redhat.com>; Shenwei Wang
> <shenwei.wang@nxp.com>; David S. Miller <davem@davemloft.net>; Eric
> Dumazet <edumazet@google.com>; Jakub Kicinski <kuba@kernel.org>; Paolo
> Abeni <pabeni@redhat.com>; Maxime Coquelin
> <mcoquelin.stm32@gmail.com>; Shawn Guo <shawnguo@kernel.org>; Sascha
> Hauer <s.hauer@pengutronix.de>; Neil Armstrong <neil.armstrong@linaro.org=
>;
> Kevin Hilman <khilman@baylibre.com>; Vinod Koul <vkoul@kernel.org>; Chen-
> Yu Tsai <wens@csie.org>; Jernej Skrabec <jernej.skrabec@gmail.com>; Samue=
l
> Holland <samuel@sholland.org>; Giuseppe Cavallaro
> <peppe.cavallaro@st.com>; Alexandre Torgue <alexandre.torgue@foss.st.com>=
;
> Jose Abreu <joabreu@synopsys.com>; Pengutronix Kernel Team
> <kernel@pengutronix.de>; Fabio Estevam <festevam@gmail.com>; dl-linux-imx
> <linux-imx@nxp.com>; Jerome Brunet <jbrunet@baylibre.com>; Martin
> Blumenstingl <martin.blumenstingl@googlemail.com>; Bhupesh Sharma
> <bhupesh.sharma@linaro.org>; Nobuhiro Iwamatsu
> <nobuhiro1.iwamatsu@toshiba.co.jp>; Simon Horman
> <simon.horman@corigine.com>; Bartosz Golaszewski
> <bartosz.golaszewski@linaro.org>; Wong Vee Khee <veekhee@apple.com>;
> Revanth Kumar Uppala <ruppala@nvidia.com>; Jochen Henneberg
> <jh@henneberg-systemdesign.com>; netdev@vger.kernel.org; linux-stm32@st-
> md-mailman.stormreply.com; linux-arm-kernel@lists.infradead.org; linux-
> kernel@vger.kernel.org; linux-amlogic@lists.infradead.org; imx@lists.linu=
x.dev;
> Frank Li <frank.li@nxp.com>
> Subject: [EXT] Re: [PATCH v2 net 2/2] net: stmmac: dwmac-imx: pause the T=
XC
> clock in fixed-link
>
> Caution: This is an external email. Please take care when clicking links =
or
> opening attachments. When in doubt, report the message using the 'Report =
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> email' button
>
>
> On Fri, Jul 28, 2023 at 04:22:19PM +0100, Russell King (Oracle) wrote:
> > On Thu, Jul 27, 2023 at 01:36:45PM -0500, Andrew Halaney wrote:
> > > I don't have any documentation for the registers here, and as you
> > > can see I'm an amateur with respect to memory ordering based on my
> > > prior comment.
> > >
> > > But you:
> > >
> > >     1. Read intf_reg_off into variable iface
> > >     2. Write the RESET_SPEED for the appropriate mode to MAC_CTRL_REG
> > >     3. wmb() to ensure that write goes through
> >
> > I wonder about whether that wmb() is required. If the mapping is
> > device-like rather than memory-like, the write should be committed
> > before the read that regmap_update_bits() does according to the ARM
> > memory model. Maybe a bit of information about where this barrier has
> > come from would be good, and maybe getting it reviewed by the
> > arm64 barrier specialist, Will Deacon. :)
> >
> > wmb() is normally required to be paired with a rmb(), but we're not
> > talking about system memory here, so I also wonder whether wmb() is
> > the correct barrier to use.
>
> Yes, I don't think wmb() is the right thing here. If you need to ensure t=
hat the
> write to MAC_CTRL_REG has taken effect, then you'll need to go through so=
me
> device-specific sequence which probably involves reading something back. =
If you
> just need things to arrive in order eventually, the memory type already g=
ives you
> that.
>
> It's also worth pointing out that udelay() isn't necessarily ordered wrt =
MMIO
> writes, so that usleep_range() might need some help as well.
> Non-relaxed MMIO reads, however, _are_ ordered against a subsequent
> udelay(), so if you add the readback then this might all work out.
>

    1. Write RESET_SPEED
    2. Write 0 to MX93_GPR_ENET_QOS_INTF_MODE_MASK
    3. usleep_range()
    4. Restore MX93_GPR_ENET_QOS_CLK_GEN_EN | iface

In the above example, if a readback after step #2 could ensure sufficient t=
ime has passed
before step #4, the wmb() here should be abandoned.

Thanks,
Shenwei

> I gave a (slightly dated) talk about some of this at ELC a while back:
>
> https://www.yo/
> utube.com%2Fwatch%3Fv%3Di6DayghhA8Q&data=3D05%7C01%7Cshenwei.wang
> %40nxp.com%7C32396fd0396e4e46975f08db8f806680%7C686ea1d3bc2b4c6fa
> 92cd99c5c301635%7C0%7C0%7C638261553857503588%7CUnknown%7CTWFp
> bGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6
> Mn0%3D%7C3000%7C%7C%7C&sdata=3DX5CQrQEVmUjYafYJ%2BzcnGXI9mhDT%
> 2BMzDazGHOcoomas%3D&reserved=3D0
>
> which might help.
>
> Will