netdev - Re: [PATCH] lan743x: Added fixed link support

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Message-Id: <F241F806-3A08-4086-9739-361538FD246B@berg-solutions.de>
Date:   Tue, 19 May 2020 18:42:33 +0200
From:   Roelof Berg <rberg@...g-solutions.de>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Bryan Whitehead <bryan.whitehead@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lan743x: Added fixed link support

Hi Andrew,

thank you for the example, your input got me further. Sorry if my e-mails made the impression that the MAC is sending MDIO on its own. It can issue MDIO but I assume it will do this only on request of the MCU.

I read the data sheets again and found what might have confused us. There is:
a) Auto Negotiation (Phy-Phy)
b) Automatic Speed Detection, ASD (Mac-Phy)
c) Automatic Duplex Detection, ADD (Mac-Phy)

My current hypothesis is: When Phy-Phy auto negotiation is done, the ASD and ADD of the MAC will implicitly catch up the new mode of the Phy on a low level (clocks, pins). A dumb silicon would need the MCU to re-configure the MAC after MDIO told the MCU about a change in the Phy mode. But this ultra smart silicon would neither need MDIO, nor an MCU to understand what’s going on on the busses :)

If this hypothesis is correct, I should change in the driver all comments that mention „auto negoriation“ to „ADD, ASD“, and future readers will not be confused anymore.

Conclusion:
- Maybe I can leave ASD and ADD even active in fixed-link scenarios, when in the device tree an empty fixed-link node is present.
- And I need to disable ASD and/or ADD only if speed and/or duplex is configured inside the fixed-link mode.

I need to verify this hypothesis.

Thank you for reviewing and sharing topics we need to consider,
Roelof

> Am 18.05.2020 um 22:34 schrieb Andrew Lunn <andrew@...n.ch>:
> 
>> I double checked the vendor documentation and according to the data
>> sheet in this device the MAC detects speed and duplex mode. It uses
>> PINs, traces clocks … Also according to an application note of the
>> vendor duplex and speed detection should be enabled in the MAC
>> registers.
> 
> In general, the MAC should not perform MDIO requests on the PHY.  The
> MAC has no access to the mutex which phylib users. So if the MAC
> directly accesses registers in the PHY, it could do it at the wrong
> time, when the PHY driver is active.
> 
> This can be particularly bad when Marvell PHYs are used. They have
> paged registers. One example is the page with the temperature sensor.
> This can be selected due to a read on the hwmon device. If the MAC
> tried to read the speed/duplex which the temperature sensor is
> selected, it would wrongly read the temperature sensor registers, not
> the link state.
> 
> There is no need for the MAC to directly access the PHY. It will get
> told what the result of auto-neg is. So please turn this off all the
> time.
> 
> 	Andrew
>