linux-kernel - Re: [PATCH] net: renesas: Fix rgmii-id delays

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Date:   Wed, 20 Oct 2021 10:53:28 +0200
From:   Köry Maincent <kory.maincent@...tlin.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Andrew Lunn <andrew@...n.ch>, netdev <netdev@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sergey Shtylyov <s.shtylyov@....ru>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
        Biju Das <biju.das.jz@...renesas.com>,
        Sergei Shtylyov <sergei.shtylyov@...il.com>,
        Adam Ford <aford173@...il.com>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Yang Yingliang <yangyingliang@...wei.com>
Subject: Re: [PATCH] net: renesas: Fix rgmii-id delays

Hello Geert, Thomas

On Tue, 19 Oct 2021 22:05:41 +0200
Geert Uytterhoeven <geert@...ux-m68k.org> wrote:

> Hi Thomas,
> 
> On Tue, Oct 19, 2021 at 5:57 PM Thomas Petazzoni
> <thomas.petazzoni@...tlin.com> wrote:
> > On Tue, 19 Oct 2021 17:41:49 +0200
> > Andrew Lunn <andrew@...n.ch> wrote:  
> > > > When people update the kernel version don't they update also the
> > > > devicetree?  
> > >
> > > DT is ABI. Driver writers should not break old blobs running on new
> > > kernels. Often the DT blob is updated with the kernel, but it is not
> > > required. It could be stored in a hard to reach place, shared with
> > > u-boot etc.  
> >
> > Right, but conversely if someone reads the DT bindings that exists
> > today, specifies phy-mode = "rgmii-rxid" or phy-mmode = "rmgii-txid",  
> 
> Today == v5.10-rc1 and later?
> 
> > this person will get incorrect behavior. Sure a behavior that is
> > backward compatible with older DTs, but a terribly wrong one when you
> > write a new DT and read the DT binding documentation. This is exactly
> > the problem that happened to us.  
> 
> If you write a new DT, you read the DT binding documentation, and
> "make dtbs_check" will inform you politely if you use the legacy/wrong
> DT (i.e. lacking "[rt]x-internal-delay-ps")?

Indeed this command will inform the missing required "[rt]x-internal-delay-ps".
I am not use to that command, as it seems to check all the devicetree each time
where I want only to check one.

> 
> The current driver is backwards-compatible with the legacy/wrong DTB.
> The current DT bindings (as of v5.10-rc1), using "[rt]x-internal-delay-ps"
> are correct.
> Or am I missing something here?

You are correct.

> 
> BTW, it's still not clear to me why the inversion would be needed.
> Cfr. Andrew's comment:
> 
> | So with rgmii-rxid, what is actually passed to the PHY? Is your
> | problem you get twice the delay in one direction, and no delay in the
> | other?

Yes, it was the problem I got.
The PHY I use have RX delay enabled by default, currently the PHY driver does
not support delay configuration, therefore I let the MAC handle TX delay. I
have stumbling over that legacy/wrong DTS on the old Kernel.

Regards,
Köry