linux-kernel - Re: [RFC PATCH v2] net: phy: Added device tree binding for dev-addr and dev-addr code check-up

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Message-ID: <CAL_JsqJGaGdEgSnnMG8C7P6=gELnuxa-oNgBgK2Yk94HcA=LFg@mail.gmail.com>
Date:   Wed, 28 Mar 2018 09:31:20 -0500
From:   Rob Herring <robh@...nel.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Vicenţiu Galanopulo <vicentiu.galanopulo@....com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "marcel@...tmann.org" <marcel@...tmann.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Madalin-cristian Bucur <madalin.bucur@....com>,
        Alexandru Marginean <alexandru.marginean@....com>
Subject: Re: [RFC PATCH v2] net: phy: Added device tree binding for dev-addr
 and dev-addr code check-up

On Tue, Mar 27, 2018 at 9:24 AM, Andrew Lunn <andrew@...n.ch> wrote:
>> > This is a 2nd MDIO address, right? Can't you just append this to reg property?
>
> Hi Rob
>
> It is a sub address.
>
> There are two different MDIO addressing schemes. Clause 22 allowed for
> 32 different addresses on an MDIO bus. Clause 45 extended that. You
> have the existing 32 addresses for a package. However, within a
> package, you can have 32 devices.

Sounds similar to functions in PCI land (which are part of reg address).

> You are supposed to be able to look are registers inside the package,
> and it will tell you which devices in the packages are in use. You can
> then look at those devices and figure out what they are using ID
> registers.
>
> However some vendors get this wrong, they don't fill in the devices in
> package information. So the generic probe code never finds them. We
> need to pass it a hint, go looking at this specific device in the
> package.

If this is a rare case and something future devices should get right,
then I'm more inclined to use 'dev-addr' rather than extending reg.

> You can mix Clause 22 and Clause 45 on the same bus. Does DT allow two
> different reg formats to be used at once? Can we have some reg
> properties with a single value, and some with two values? I thought
> #address-cells = <1> means there should be a single address in reg.

#address-cells is how many cells (aka u32) it takes to store an
address, not how many addresses you have. The number of addresses is
(sizeof(reg) / 4) / (#address-cells + #size-cells). So yes, you can
have different number of addresses for each device, but you can't have
different sizes of addresses (e.g. 32-bit and 64-bit) in one bus. But
I think in this case it would logically be 1 address with 2 cells
because it is the port and device together that make up the address.
If you did that, you'd have to define how to express a clause 22
device with 2 cells. You could either set a high bit in the first cell
to indicate clause 45 address or use an illegal device address in the
2nd cell.

However, the MDIO core would need to handle 2 address cells. If more
than 1 address cell is an error currently, that causes a compatibility
problem with new dtb and older kernels (but could be addressed with
stable updates).

Rob