linux-kernel - Re: [PATCH net-next 2/2] net: mdio: add amlogic gxl mdio mux support

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Message-ID: <1jcz79wlgc.fsf@starbuckisacylon.baylibre.com>
Date:   Fri, 20 Jan 2023 11:16:20 +0100
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        linux-amlogic@...ts.infradead.org,
        Kevin Hilman <khilman@...libre.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Da Xue <da@...sconfused.com>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 2/2] net: mdio: add amlogic gxl mdio mux support


On Thu 19 Jan 2023 at 18:17, Andrew Lunn <andrew@...n.ch> wrote:

>> >> +
>> >> +	/* Set the internal phy id */
>> >> +	writel_relaxed(FIELD_PREP(REG2_PHYID, 0x110181),
>> >> +		       priv->regs + ETH_REG2);
>> >
>> > So how does this play with what Heiner has been reporting recently?
>> 
>> What Heiner reported recently is related to the g12 family, not the gxl
>> which this driver address.
>> 
>> That being said, the g12 does things in a similar way - the glue
>> is just a bit different:
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/mdio/mdio-mux-meson-g12a.c?h=v6.2-rc4#n165
>> 
>> > What is the reset default? Who determined this value?
>> 
>> It's the problem, the reset value is 0. That is why GXL does work with the
>> internal PHY if the bootloader has not initialized it before the kernel
>> comes up ... and there is no guarantee that it will.
>> 
>> The phy id value is arbitrary, same as the address. They match what AML
>> is using internally.
>
> Please document where these values have come from. In the future we
> might need to point a finger when it all goes horribly wrong.
>

OK

>> They have been kept to avoid making a mess if a vendor bootloader is
>> used with the mainline kernel, I guess.
>> 
>> I suppose any value could be used here, as long as it matches the value
>> in the PHY driver:
>> 
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/phy/meson-gxl.c?h=v6.2-rc4#n253
>
> Some Marvell Ethernet switches with integrated PHYs have IDs with the
> vendor part set to Marvell, but the lower part is 0. The date sheet
> even says this is deliberate, you need to look at some other register
> in the switches address space to determine what the part is. That
> works O.K in the vendor crap monolithic driver, but not for Linux
> which separates the drivers up. So we have to intercept the reads and
> fill in the lower part. And we have no real knowledge if the PHYs are
> all the same, or there are differences. So we put in the switch ID,
> and the PHY driver then has an entry per switch. That gives us some
> future wiggle room if we find the PHYs are actually different.
>
> Is there any indication in the datasheets that the PHY is the exact
> same one as in the g12? Are we really safe to reuse this value between
> different SoCs?

There is zero information about the PHY in the datasheet.
The gxl and g12 don't use the same ID values.
The PHY ip is very similar but slightly different between the 2.
(see https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/phy/meson-gxl.c)

My guess is the g12 as another version of the IP, with some bug fixed.
The integration (clocking scheme mostly) is also different, which is why
the mux/glue is different.

>
> I actually find it an odd feature. Does the datasheet say anything
> about Why you can set the ID in software? The ID describes the
> hardware, and software configuration should not be able to change the
> hardware in any meaningful way.

Again, zero information. 
It is a bought IP (similar to the Rockchip judging by the PHY driver).
I'm not surprised the provider of the IP would make the ID
easy to configure. AML chose to keep that configurable through the glue,
instead of fixing it. This is how it is.

>
>> >> +	/* Enable the internal phy */
>> >> +	val |= REG3_PHYEN;
>> >> +	writel_relaxed(val, priv->regs + ETH_REG3);
>> >> +	writel_relaxed(0, priv->regs + ETH_REG4);
>> >> +
>> >> +	/* The phy needs a bit of time to come up */
>> >> +	mdelay(10);
>> >
>> > What do you mean by 'come up'? Not link up i assume. But maybe it will
>> > not respond to MDIO requests?
>> 
>> Yes this MDIO multiplexer is also the glue that provides power and
>> clocks to the internal PHY. Once the internal PHY is selected, it needs
>> a bit a of time before it is usuable. 
>
> O.K, please reword it to indicate power up, not link up.
>

Sure

>      Andrew