linux-kernel - Re: [PATCH V11 2/4] ptp: Added a clock that uses the eTSEC found on the MPC85xx.

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Message-ID: <20110223175459.GH14597@angua.secretlab.ca>
Date:	Wed, 23 Feb 2011 10:54:59 -0700
From:	Grant Likely <grant.likely@...retlab.ca>
To:	Scott Wood <scottwood@...escale.com>
Cc:	Richard Cochran <richardcochran@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rodolfo Giometti <giometti@...ux.it>,
	Arnd Bergmann <arnd@...db.de>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-api@...r.kernel.org, devicetree-discuss@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Paul Mackerras <paulus@...ba.org>,
	John Stultz <john.stultz@...aro.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, netdev@...r.kernel.org,
	Mike Frysinger <vapier@...too.org>,
	Christoph Lameter <cl@...ux.com>,
	linuxppc-dev@...ts.ozlabs.org, David Miller <davem@...emloft.net>,
	linux-arm-kernel@...ts.infradead.org,
	Krzysztof Halasa <khc@...waw.pl>
Subject: Re: [PATCH V11 2/4] ptp: Added a clock that uses the eTSEC found
 on the MPC85xx.

On Wed, Feb 23, 2011 at 11:26:12AM -0600, Scott Wood wrote:
> On Wed, 23 Feb 2011 09:50:58 -0700
> Grant Likely <grant.likely@...retlab.ca> wrote:
> 
> > On Wed, Feb 23, 2011 at 11:38:17AM +0100, Richard Cochran wrote:
> > > +
> > > +* Gianfar PTP clock nodes
> > > +
> > > +General Properties:
> > > +
> > > +  - compatible   Should be "fsl,etsec-ptp"
> > 
> > Should specify an *exact* part; ie: "fsl,mpc8313-etsec-ptp" instead of
> > trying to define a generic catchall.  The reason is that the same
> > marketing name can end up getting applied to a wide range of parts.
> > 
> > Instead, choose one specific device to stand in as the 'common'
> > implementation and get all parts with the same core to claim
> > compatibility with it.  ie: a p2020 might have:
> > 
> > 	compatible = "fsl,mpc2020-etsec-ptp", "fsl,mpc8313-etsec-ptp";
> 
> eTSEC is versioned, that's more reliable than the chip name since chips
> have revisions (rev 2.1 of mpc8313 has eTSEC 1.6, not sure about previous
> revs of mpc8313).  Logic blocks can be and have been uprevved between one
> revision of a chip to the next.  I think "fsl,mpc8313rev2.1-etsec-ptp"
> would be taking things a bit too far (and there could be board-level bugs
> too...).
> 
> If you really need to know the exact SoC you're on, look in SVR (which
> will provide revision info as well).  Isn't the device tree for things that
> can't be probed?

This is far more about the binding than it is about the chip revision.
When documenting a binding it makes far more sense to anchor it to a
specific implementation than to try and come up with a 'generic'
catchall.  A new binding means new compatible value and dropping any
claims of being compatible with the old.

It is not about the logic block version, particularly when it is
detectable by the driver as you say.

> 
> The eTSEC revision is probeable as well, but due the way PTP is described as
> a separate node, the driver doesn't have straightforward access to those
> registers.

Ignorant question: Should the ptp be described as a separate node?

> 
> Insisting on an explicit chip also encourages people to claim compatibility
> with that chip without ensuring that it really is fully compatible.

In practise, I've not seen this to be an issue.

g.


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