Re: [PATCH] DT: add MDIO node for FMan node

On Thu, 2014-11-13 at 02:27 -0600, Xie Shaohui-B21989 wrote:

Best Regards, 
Shaohui Xie

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-----Original Message-----
From: Wood Scott-B07421
Sent: Thursday, November 13, 2014 4:04 PM
To: Xie Shaohui-B21989
Cc: Liberman Igal-B31950; <redacted>;
devicetree@vger.kernel.org; Medve Emilian-EMMEDVE1
Subject: Re: [PATCH] DT: add MDIO node for FMan node

On Thu, 2014-11-13 at 02:02 -0600, Xie Shaohui-B21989 wrote:

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Best Regards,
Shaohui Xie

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-----Original Message-----
From: Wood Scott-B07421
Sent: Thursday, November 13, 2014 3:15 PM
To: Xie Shaohui-B21989
Cc: Liberman Igal-B31950; <redacted>;
devicetree@vger.kernel.org; Medve Emilian-EMMEDVE1
Subject: Re: [PATCH] DT: add MDIO node for FMan node

On Thu, 2014-11-13 at 01:11 -0600, Xie Shaohui-B21989 wrote:

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-----Original Message-----
From: Wood Scott-B07421
Sent: Thursday, November 13, 2014 2:17 PM
To: Xie Shaohui-B21989
Cc: Liberman Igal-B31950; <redacted>;
devicetree@vger.kernel.org; Medve Emilian-EMMEDVE1
Subject: Re: [PATCH] DT: add MDIO node for FMan node

On Wed, 2014-11-12 at 07:40 -0600, Xie Shaohui-B21989 wrote:

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-----Original Message-----
From: Wood Scott-B07421
Sent: Wednesday, November 12, 2014 1:38 AM
To: Xie Shaohui-B21989
Cc: Liberman Igal-B31950; <redacted>;
devicetree@vger.kernel.org; Medve Emilian-EMMEDVE1
Subject: Re: [PATCH] DT: add MDIO node for FMan node

On Tue, 2014-11-11 at 04:32 -0600, Xie Shaohui-B21989 wrote:

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-----Original Message-----
From: Wood Scott-B07421
Sent: Tuesday, November 11, 2014 8:23 AM
To: <redacted>
Cc: <redacted>;
devicetree@vger.kernel.org; Medve Emilian-EMMEDVE1; Xie
Shaohui-B21989
Subject: Re: [PATCH] DT: add MDIO node for FMan node

On Tue, 2014-11-04 at 19:56 +0800, shh.xie@gmail.com

wrote:

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From: Shaohui Xie <redacted>

This binding is for FMan MDIO, it covers FMan v2 & FMan

v3.

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Signed-off-by: Shaohui Xie <redacted>
---
based on http://patchwork.ozlabs.org/patch/390351/
for 'next' of

git://git.kernel.org/pub/scm/linux/kernel/git/scottwood/linux.

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git

Are there any other FMan pieces that are missing from
the above

patch?

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[S.H] I'm adding Igal for this comment.

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+- bus-frequency
+		Usage: optional
+		Value type: <u32>
+		Definition: Default MDIO bus clock speed.

Use clocks/clock-names

[S.H] The MDIO uses Fman clock and divides it to a proper
value which

is specified by this property.

Use clocks/clock-names to describe that relationship.

[S.H] The MDIO node is sub-node and embedded in Fman node, the
clocks/clock-names is provided by Fman node, should repeat
them in MDIO node? For the default MDIO bus clock speed, maybe
"clock-

ranges"

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should be used?

It's a different clock.  You wouldn't be repeating.  If it's
derived from the FMan clock, then maybe you don't need anything
here (does the driver know what the divider is, or would that
need to be specified in the device tree?), but no more
clock-frequency/bus-

frequency properties.

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[S.H] The purpose here is to get a specific clock frequency,
driver to

use it to calculate the divider.

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Then the Fman clock can be divided to the frequency.

Oh, so this is stating a desired frequency and not something that
already exists?  What determines this frequency?  Is it based on
board design, or just on the MDIO standard, etc?  I'm wondering if
the device tree is the right place for it.

[S.H] Yes, a desired frequency which is different with MDIO standard.

I'm not sure what you mean by "different with".  Do you mean "different
from"?  What does the standard say about frequency?

[S.H] The standard MDIO frequency is 2.5MHz.  But a different one is desired.

Why is a different frequency desired?

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The Fman clock and the divider determines this frequency.
Since Fman clock is different on different SoCs, so specify the
desired frequency, then to get the proper divider.

Why does the fman clock being different mean the mdio clock should be
different?

[S.H] The mdio clock should be same, a different divider should be used 
to match the Fman clock to make sure the mdio clock kept same. So to specify
The desired mdio frequency, then to get proper divider.

What I'm trying to determine is why the MDIO frequency wouldn't always
be the same, and thus could be hardcoded in the driver rather than in
the device tree.

-Scott