linux-kernel - Re: [PATCH 3/3] ARM: zynq: DT: Add Ethernet phys

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Date:	Mon, 25 Aug 2014 13:21:24 -0700
From:	Florian Fainelli <f.fainelli@...il.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
CC:	Sören Brinkmann <soren.brinkmann@...inx.com>,
	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Michal Simek <michal.simek@...inx.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Kumar Gala <galak@...eaurora.org>,
	Andreas Färber <afaerber@...e.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 3/3] ARM: zynq: DT: Add Ethernet phys

On 08/25/2014 10:46 AM, Jason Gunthorpe wrote:
> On Fri, Aug 22, 2014 at 01:47:09PM -0700, Florian Fainelli wrote:
> 
>>>  - the ID based strings seem to be not needed since, IIUC, the core
>>>    reads the ID from the PHY and uses it, so I just left it out not
>>>    trying to figure out how to obtain the correct ID
>>
>> It is not needed, but it is one way to specify a PHY device if you do
>> not know what compatible string to use instead.
> 
> No, it is a way to specify a PHY device if the kernel can't auto probe
> the Phy ID.
> 
> Last I checked, the kernel doesn't support plain text compatible
> strings for phys - everything is driven on the phy id, either auto
> probed or specified in the DT.

That's right. Some PHY drivers might be relying on specific compatible
strings though, but not the core PHY library that probes and maps a
driver to a PHY node.

> 
>>>  - the marvell compatible strings are used in our vendor tree. They
>>>    aren't used anywhere but in our vendor tree. I though keeping them is
>>>    nice since it identifies the PHY fully. And in case that level of
>>>    detail is needed at some point it is already there.
>>
>> And this is the recommended way to do it in case we ever need to key a
>> software decision based on the hardware.
> 
> All compatible strings need to be documented.
> 
> .. and they need to encode more information than you get from the phy
> id - die revsision, package option, functional options, voltage
> codes. Etc.
> 
> .. and they actually need to be *right*

Agreed.

> 
> An example: The kernel reports 88E1318S for all four chips in that
> family, AFAIK you have to read the package marking to figure out which
> you have (it is the same die, with options switched on/off at
> packaging time). People have already posted patches trying to
> helpfully add a 'marvell,88E1318S' compatible string based on kernel
> output. Except it is wrong, it isn't actually the '8S version in the
> HW.
> 
> Even worse, Marvell has a whole series of socket compatible phys. Just
> because the board the DT author looked at has a '318, doesn't mean
> that every board ever made will. We've actually already been switching
> between the 318 and 318S for production depending on which has part
> availability.
> 
> Basically: don't try to override self-discoverable hardware in DT
> without a really good reason.

I think that's a very good point, at the very least let's use a
compatible string that contains the full 32-bits PHY OUI.

Thanks
--
Florian
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