netdev - Re: [PATCH v3 0/7] mv643xx.c: Add basic device tree support.

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Message-ID: <5023D577.8090001@codethink.co.uk>
Date:	Thu, 09 Aug 2012 16:21:27 +0100
From:	Ian Molton <ian.molton@...ethink.co.uk>
To:	Arnd Bergmann <arnd@...db.de>
CC:	linux-arm-kernel@...ts.infradead.org,
	thomas.petazzoni@...e-electrons.com, andrew@...n.ch,
	netdev@...r.kernel.org, devicetree-discuss@...ts.ozlabs.org,
	ben.dooks@...ethink.co.uk, dale@...nsworth.org,
	linuxppc-dev@...ts.ozlabs.org, David Miller <davem@...emloft.net>
Subject: Re: [PATCH v3 0/7] mv643xx.c: Add basic device tree support.

On 09/08/12 12:43, Arnd Bergmann wrote:
>  On 08/08/12 14:19, Ian Molton wrote:
>  > On 08/08/12 13:39, Arnd Bergmann wrote:
>  >> On Wednesday 08 August 2012, Ian Molton wrote:
>  >>> This method would require a small amount of rework in the driver to
>  >>> set up <n> ports, rather than just one.
>  >> This looks quite nice, but it is still very much incompatible with the
>  >> existing binding. Obviously we can abandon an existing binding and
>  >> introduce a second one for the same hardware, but that should not
>  >> be taken lightly.
>  > Fair, however the existing users aren't anywhere near as
>  > numerous as the new ones.
>
> Depends on how you count the numbers. I see at least three machines
> supported in the kernel with the old binding and none with the new one
> so far ;-)

I'm curious as to how any of those actually work, given the
apparent total lack of a mv64360-mdio device binding...
>  > As you can see, instead of putting port1 at +0x1700 or so,
>  > marvell have overlapped the register files - in fact, doubly
>  > so, since port1 + 0x1080 is right in the middle of
>  > (port0 + 0x1000) -> (port0 + 0x16ff), so one cant simply map two
>  > sets of regs like 0x0000->0x03ff and 0x1000->0x16ff for port one
>  > either.
>
> This could theoretically be dealt with by having 5 register ranges

I make that three...

> per device, but that would cause extra overhead and also be
> incompatible with the existing binding.

Indeed.

>  I think showing one
> parent device with children at address 0, 1 and 2 is ok.
Is it acceptable for the child devices to directly access the
parents register space? because there would be no other
way for that to work.

>  The driver
> already knows all those offsets and they are always the same
> for all variants of mv643xx, right?
Yes, but its not clean. And no amount of refactoring is
really going to make a nice driver that also fits the ancient
(and badly thought out) OF bindings.

If we have to break things, we can at least go for a nice
clean design, surely?

The ports arent really child devices of the MAC. The MAC
just has 3 ports.

Luckily, it looks like the existing users don't actually use
the device tree to set up the driver at all, preferring to
translate their D-T bindings to calls to
platform_device_register() so all we'd need to do to
support them is completely ignore them.

We're going to have to maintain a legacy
platform_device -> DT bindings hack somewhere anyway,
at least until the remaining other users of the driver
convert to D-T.

-Ian
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