Date:   Wed, 1 Dec 2021 09:02:05 +0000
From:   "Russell King (Oracle)" <linux@armlinux.org.uk>
To:     Andrew Lunn <andrew@lunn.ch>
Cc:     Heiner Kallweit <hkallweit1@gmail.com>,
        Yinbo Zhu <zhuyinbo@loongson.cn>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Nick Desaulniers <ndesaulniers@google.com>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kbuild@vger.kernel.org
Subject: Re: [PATCH v2 1/2] modpost: file2alias: fixup mdio alias garbled
 code in modules.alias
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Sender: Russell King (Oracle) <linux@armlinux.org.uk>
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On Wed, Dec 01, 2021 at 01:38:53AM +0100, Andrew Lunn wrote:
> > However, this won't work for PHY devices created _before_ the kernel
> > has mounted the rootfs, whether or not they end up being used. So,
> > every PHY mentioned in DT will be created before the rootfs is mounted,
> > and none of these PHYs will have their modules loaded.
> 
> Hi Russell
> 
> I think what you are saying here is, if the MAC or MDIO bus driver is
> built in, the PHY driver also needs to be built in?
> 
> If the MAC or MDIO bus driver is a module, it means the rootfs has
> already been mounted in order to get these modules. And so the PHY
> driver as a module will also work.

Yes, because the module loading is performed by phy_device_create() when
it calls phy_request_driver_module(), which will happen when either the
MDIO bus is scanned or the DT is parsed for the PHY nodes.

> > I believe this is the root cause of Yinbo Zhu's issue.
> 
> You are speculating that in Yinbo Zhu case, the MAC driver is built
> in, the PHY is a module. The initial request for the firmware fails.

s/firmware/module/ and it could also be the MDIO bus driver that is
built in.

> Yinbo Zhu would like udev to try again later when the modules are
> available.

I think so - it's speculation because it seems quite difficult to find
out detailed information.

> > What we _could_ do is review all device trees and PHY drivers to see
> > whether DT modaliases are ever used for module loading. If they aren't,
> > then we _could_ make the modalias published by the kernel conditional
> > on the type of mdio device - continue with the DT approach for non-PHY
> > devices, and switch to the mdio: scheme for PHY devices. I repeat, this
> > can only happen if no PHY drivers match using the DT scheme, otherwise
> > making this change _will_ cause a regression.
> 
> Take a look at
> drivers/net/mdio/of_mdio.c:whitelist_phys[] and the comment above it.
> 
> So there are some DT blobs out there with compatible strings for
> PHYs. I've no idea if they actually load that way, or the standard PHY
> mechanism is used.

Well, this suggests we have no instances - if none of our modules
contain a DT table to match a PHY-driver, then we should be pretty
safe.

$ grep phy_driver drivers/net -rl | xargs grep 'MODULE_ALIAS\|MODULE_DEVICE.*of'
drivers/net/phy/xilinx_gmii2rgmii.c:MODULE_DEVICE_TABLE(of, xgmiitorgmii_of_match);
drivers/net/mdio/mdio-moxart.c:MODULE_DEVICE_TABLE(of, moxart_mdio_dt_ids);
drivers/net/dsa/mt7530.c:MODULE_DEVICE_TABLE(of, mt7530_of_match);

All three look to be false hits - none are phy drivers themselves, they
just reference "phy_driver". So, I think we can say that we have no
instances of PHY driver being matched using DT in net-next in
drivers/net. Hopefully, there aren't any PHY drivers elsewhere in the
kernel tree.

That is not true universally for all MDIO though - as
xilinx_gmii2rgmii.c clearly shows. That is a MDIO driver which uses DT
the compatible string to do the module load. So, we have proof there
that Yinbo Zhu's change will definitely cause a regression which we
can not allow.

-- 
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