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Date:   Fri, 22 Jan 2021 12:20:27 +0100
From:   Michael Walle <michael@walle.cc>
To:     Claudiu.Beznea@microchip.com
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Nicolas.Ferre@microchip.com, davem@davemloft.net
Subject: Re: [PATCH] net: macb: ignore tx_clk if MII is used
In-Reply-To: <1bde9969-8769-726b-02cb-a1fcded0cd74@microchip.com>
References: <20210120194303.28268-1-michael@walle.cc>
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Am 2021-01-22 10:10, schrieb Claudiu.Beznea@microchip.com:
> On 21.01.2021 11:41, Michael Walle wrote:
>> EXTERNAL EMAIL: Do not click links or open attachments unless you know 
>> the
>> content is safe
>> 
>> Hi Claudiu,
>> 
>> Am 2021-01-21 10:19, schrieb Claudiu.Beznea@microchip.com:
>>> On 20.01.2021 21:43, Michael Walle wrote:
>>>> EXTERNAL EMAIL: Do not click links or open attachments unless you 
>>>> know
>>>> the content is safe
>>>> 
>>>> If the MII interface is used, the PHY is the clock master, thus 
>>>> don't
>>>> set the clock rate. On Zynq-7000, this will prevent the following
>>>> warning:
>>>>   macb e000b000.ethernet eth0: unable to generate target frequency:
>>>> 25000000 Hz
>>>> 
>>> 
>>> Since in this case the PHY provides the TX clock and it provides the
>>> proper
>>> rate based on link speed, the MACB driver should not handle the
>>> bp->tx_clk
>>> at all (MACB driver uses this clock only for setting the proper rate 
>>> on
>>> it
>>> based on link speed). So, I believe the proper fix would be to not 
>>> pass
>>> the
>>> tx_clk at all in device tree. This clock is optional for MACB driver.
>> 
>> Thanks for looking into this.
>> 
>> I had the same thought. But shouldn't the driver handle this case
>> gracefully?
>> I mean it does know that the clock isn't needed at all.
> 
> Currently it may knows that by checking the bp->tx_clk. Moreover the 
> clock
> could be provided by PHY not only for MII interface.

That doesn't make this patch wrong, does it? It just doesn't cover
all use cases (which also wasn't covered before).

> Moreover the IP has the bit "refclk" of register at offset 0xc (userio)
> that tells it to use the clock provided by PHY or to use one internal 
> to
> the SoC. If a SoC generated clock would be used the IP logic may have 
> the
> option to do the proper division based on link speed (if IP has this 
> option
> enabled then this should be selected in driver with capability
> MACB_CAPS_CLK_HW_CHG).
> 
> If the clock provided by the PHY is the one to be used then this is
> selected with capability MACB_CAPS_USRIO_HAS_CLKEN. So, if the change 
> you
> proposed in this patch is still imperative then checking for this
> capability would be the best as the clock could be provided by PHY not 
> only
> for MII interface.

Fair enough, but this register doesn't seem to be implemented on
Zynq-7000. Albeit MACB_CAPS_USRIO_DISABLED isn't defined for the
Zynq MACB. It isn't defined in the Zynq-7000 reference manual and
you cannot set any bits:

=> mw 0xE000B00C 0xFFFFFFFF
=> md 0xE000B00C 1
e000b00c: 00000000

Also please note, that tx_clk may be an arbitrary clock which doesn't
necessarily need to be the clock which is controlled by CLK_EN. Or
am I missing something here?

-michael

>> Ususually that
>> clock
>> is defined in a device tree include. So you'd have to redefine that 
>> node
>> in
>> an actual board file which means duplicating the other clocks.
>> 
>> -michael