linux-kernel - Re: [EXT] Re: [PATCH v3 net 2/2] net: stmmac: dwmac-imx: pause the TXC clock in fixed-link

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Message-ID: <ZMk/xqRP67zXHNrf@shell.armlinux.org.uk>
Date:   Tue, 1 Aug 2023 18:24:22 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Shenwei Wang <shenwei.wang@....com>
Cc:     Johannes Zink <j.zink@...gutronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Kevin Hilman <khilman@...libre.com>,
        Vinod Koul <vkoul@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Samuel Holland <samuel@...lland.org>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        dl-linux-imx <linux-imx@....com>,
        Jerome Brunet <jbrunet@...libre.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Bhupesh Sharma <bhupesh.sharma@...aro.org>,
        Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@...hiba.co.jp>,
        Simon Horman <simon.horman@...igine.com>,
        Andrew Halaney <ahalaney@...hat.com>,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>,
        Wong Vee Khee <veekhee@...le.com>,
        Revanth Kumar Uppala <ruppala@...dia.com>,
        Jochen Henneberg <jh@...neberg-systemdesign.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-stm32@...md-mailman.stormreply.com" 
        <linux-stm32@...md-mailman.stormreply.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-amlogic@...ts.infradead.org" 
        <linux-amlogic@...ts.infradead.org>,
        "imx@...ts.linux.dev" <imx@...ts.linux.dev>,
        Frank Li <frank.li@....com>
Subject: Re: [EXT] Re: [PATCH v3 net 2/2] net: stmmac: dwmac-imx: pause the
 TXC clock in fixed-link

On Tue, Aug 01, 2023 at 05:06:46PM +0000, Shenwei Wang wrote:
> > On Tue, Aug 01, 2023 at 02:47:46PM +0200, Johannes Zink wrote:
> > > Hi Shenwei,
> > >
> > > thanks for your patch.
> > >
> > > On 7/31/23 18:19, Shenwei Wang wrote:
> > > > When using a fixed-link setup, certain devices like the SJA1105
> > > > require a small pause in the TXC clock line to enable their internal
> > > > tunable delay line (TDL).
> > >
> > > If this is only required for some devices, is it safe to enforce this
> > > behaviour unconditionally for any kind of fixed link devices connected
> > > to the MX93 EQOS or could this possibly break for other devices?
> >
> > This same point has been raised by Andrew Halaney in message-id
> > <4govb566nypifbtqp5lcbsjhvoyble5luww3onaa2liinboguf@...ihys6vhrg>
> > and Fabio Estevam in message-id
> >
> > <CAOMZO5ANQmVbk_jy7qdVtzs3716FisT2c72W+3WZyu7FoAochw@...l.gmail.
> > com>
> > but we don't seem to have any answer for it.
> >
> Hi Russell,
> 
> I hope you have thoroughly read all of my earlier responses, as I believe I already addressed this question.
> I'm happy to clarify further, but kindly avoid unsubstantiated comments.
> 
> https://lore.kernel.org/imx/20230727152503.2199550-1-shenwei.wang@nxp.com/T/#m08da3797a056d4d8ea4c1d8956b445ae967e7cfa
> " Yes, that's the purpose because it won't hurt even the other side is not SJA1105."

So, why not include the answer in the commit message given that you've
had to answer it several times already?

> > Also, the patch still uses wmb() between the write and the delay, and as Will
> > Deacon pointed out in his message, message-id
> > <20230728153611.GH21718@...lie-the-truck>
> > this is not safe, yet still a new version was sent.
> >
> 
> Can we conclude that even without the wmb() here, the desired delay time between
> operations can still be ensured?

How did you come to that conclusion? I see no further discussion after
I raised this, Will replied, and you suggested a read-back. However,
that isn't what you've implemented on v3, you've gone back to what
looks like the original code in v2 which brought up this question - and
Will indicated it was _unsafe_.

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