netdev - Re: Linux mvneta driver unable to read MAC address from HW

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Date:   Tue, 13 Oct 2020 08:35:54 +0200
From:   Thomas Petazzoni <thomas.petazzoni@...tlin.com>
To:     Ezra Buehler <ezra@...yb.ch>
Cc:     netdev@...r.kernel.org, Stefan Roese <sr@...x.de>,
        linux-arm-kernel@...ts.infradead.org, u-boot@...ts.denx.de
Subject: Re: Linux mvneta driver unable to read MAC address from HW

Hello Ezra,

On Sat, 10 Oct 2020 18:41:24 +0200
Ezra Buehler <ezra@...yb.ch> wrote:

> I am running Debian buster (Linux 4.19.146) on a Synology DS214+ NAS
> (Marvell ARMADA XP). Unfortunately, I end up with random MAC addresses
> for the two Ethernet interfaces after boot. (Also with Debian sid, Linux
> 5.4.0.)
> 
> Since commit 8cc3e439ab92 ("net: mvneta: read MAC address from hardware
> when available") the mvneta Linux driver reads the MVNETA_MAC_ADDR_*
> registers when no MAC address is provided by the DT. In my case, only
> zeros are read, causing the driver to fall back to a random address. I
> was able to verify that the registers are correctly written by the
> bootloader by reading out the registers in the U-Boot prompt.
> 
> As a workaround, I now specify the MAC addresses in the DT. However, I
> would prefer not to do that. Also, it would be nice to get to the bottom
> of this.
> 
> Could it be, that for some reason, the clock of the MAC is removed
> either by U-Boot or Linux during boot?

I suspect you have the mvneta driver as a module ? If this is the case,
then indeed, the MAC address is lost because Linux turns of all unused
clocks at the end of the boot. When the driver is built-in, there is a
driver adding a reference to the clock before all unused clocks are
disabled. When the driver is compiled as a module, this does not
happen. So indeed, the correct solution here is to have U-Boot pass the
MAC address in the Device Tree.

Best regards,

Thomas Petazzoni
-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com