linux-kernel - Re: [RFC PATCH net-next 1/3] net: phy: don't bind genphy in phy_attach_direct if the specific driver defers probe

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Date:   Thu, 2 Sep 2021 13:33:57 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Andrew Lunn <andrew@...n.ch>,
        "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        linux-kernel@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Alvin Šipraga <alsi@...g-olufsen.dk>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        Len Brown <lenb@...nel.org>
Subject: Re: [RFC PATCH net-next 1/3] net: phy: don't bind genphy in
 phy_attach_direct if the specific driver defers probe



On 9/2/2021 12:51 PM, Andrew Lunn wrote:
> On Thu, Sep 02, 2021 at 07:50:16PM +0100, Russell King (Oracle) wrote:
>> On Thu, Sep 02, 2021 at 01:50:51AM +0300, Vladimir Oltean wrote:
>>> diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
>>> index 52310df121de..2c22a32f0a1c 100644
>>> --- a/drivers/net/phy/phy_device.c
>>> +++ b/drivers/net/phy/phy_device.c
>>> @@ -1386,8 +1386,16 @@ int phy_attach_direct(struct net_device *dev, struct phy_device *phydev,
>>>   
>>>   	/* Assume that if there is no driver, that it doesn't
>>>   	 * exist, and we should use the genphy driver.
>>> +	 * The exception is during probing, when the PHY driver might have
>>> +	 * attempted a probe but has requested deferral. Since there might be
>>> +	 * MAC drivers which also attach to the PHY during probe time, try
>>> +	 * harder to bind the specific PHY driver, and defer the MAC driver's
>>> +	 * probing until then.
>>>   	 */
>>>   	if (!d->driver) {
>>> +		if (device_pending_probe(d))
>>> +			return -EPROBE_DEFER;
>>
>> Something else that concerns me here.
>>
>> As noted, many network drivers attempt to attach their PHY when the
>> device is brought up, and not during their probe function.
> 
> Yes, this is going to be a problem. I agree it is too late to return
> -EPROBE_DEFER. Maybe phy_attach_direct() needs to wait around, if the
> device is still on the deferred list, otherwise use genphy. And maybe
> a timeout and return -ENODEV, which is not 100% correct, we know the
> device exists, we just cannot drive it.

Is it really going to be a problem though? The two cases where this will 
matter is if we use IP auto-configuration within the kernel, which this 
patchset ought to be helping with, if we are already in user-space and 
the PHY is connected at .ndo_open() time, there is a whole lot of things 
that did happen prior to getting there, such as udevd using modaliases 
in order to load every possible module we might, so I am debating 
whether we will really see a probe deferral at all.

> 
> Can we tell we are in the context of a driver probe? Or do we need to
> add a parameter to the various phy_attach API calls to let the core
> know if this is probe or open?

Actually we do the RTNL lock will be held during ndo_open and it won't 
during driver probe.

> 
> This is more likely to be a problem with NFS root, with the kernel
> bringing up an interface as soon as its registered. userspace bringing
> up interfaces is generally much later, and udev tends to wait around
> until there are no more driver load requests before the boot
> continues.

See my point above, with Vladimir's change, we should have fw_devlink do 
its job such that by the time the network interface is needed for IP 
auto-configuration, all of its depending resources should also be ready, 
would not they?
-- 
Florian