Re: [PATCH] [v2] power/fsl: add MDIO dt binding for FMan

Hello Shao-Hui,


On 12/21/2014 08:31 PM, Xie Shaohui-B21989 wrote:

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On Fri, 2014-12-19 at 01:23 -0600, Xie Shaohui-B21989 wrote:

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-----Original Message-----
From: Wood Scott-B07421
Sent: Friday, December 19, 2014 6:01 AM
To: Xie Shaohui-B21989
Cc: linuxppc-dev@lists.ozlabs.org; devicetree@vger.kernel.org; Medve
Emilian- EMMEDVE1; Liberman Igal-B31950
Subject: Re: [PATCH] [v2] power/fsl: add MDIO dt binding for FMan

On Thu, 2014-12-18 at 06:53 -0600, Xie Shaohui-B21989 wrote:

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Ping.

Best Regards,
Shaohui Xie

I can't put patches in my -next until the merge window closes.

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+EXAMPLE
+
+Example for FMan v2 external MDIO:
+
+mdio@f1000 {
+	compatible = "fsl,fman-xmdio";
+	reg = <0xf1000 0x1000>;
+	bus-frequency = <20000>;
+};

So the bus frequency is only 20 KHz?  Or is the unit supposed
to be something other than Hz?

[S.H] it's only an example, it could be different on real SoCs,
but they always lower than the standard one, The standard one is
2.5MHz, I have

to use Hz for it.

Is there any SoC for which 20 kHz is the right frequency?  I just
want to make sure the example is realistic.

[S.H] the clock divider has a limitation that the MAX value it can get
on Fman v2 is 255 (0xff, 8 bits), On Fman v3 is 511(0x1ff, 9 bits).

So the lowest frequency on Fman v2 is: Fman_clock / (2 * 255), On Fman
v3 is: Fman_clock / ((2 * 511) + 1).

Take default Fman frequency setting from SDK1.7 as example, the lowest
clock used for Fman v2 is 581MHz, The lowest clock for Fman v3 is 600MHz.

Then the lowest bus frequency can get is:
Fman v2: ~1140KHz
Fman v3: ~587KHz

20KHz is not practice, we don't have a suggested value in errata document.
For this example, should I post a new version with a value like 1200KHz?

This is different from how you described the problem before.  If the limitation
is on the divider, rather than the absolute bus frequency, then specifiy the max
divider.  Or better, since according to the above this correlates with fman
version, just have the driver know what the max divider is for each fman version.

[S.H] The problem is not the divider has limitation, the problem is a different bus frequency 
Is needed which is lower than the standard, but due to the divider limitation, the lowest
bus frequency also has limitation. i.e. we need to use the divider to get a lower frequency,
but how much lower the value could be is restricted by the divider limitation.

For the purpose of an example in the binding document, I suggest we just
stick with the IEEE standard frequency. We can continue this
conversation about errata handling when we submit the code relevant to
this binding (and the FMan v3 support)


Cheers,