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

From: "Russell King (Oracle)" <[email-masked]>
Date: Thu, 2 Sep 2021 19:50:16 +0100

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.

Taking a driver at random:

drivers/net/ethernet/renesas/sh_eth.c

sh_eth_phy_init() calls of_phy_connect() or phy_connect(), which
ultimately calls phy_attach_direct() and propagates the error code
via an error pointer.

sh_eth_phy_init() propagates the error code to its caller,
sh_eth_phy_start(). This is called from sh_eth_open(), which
probagates the error code. This is called from .ndo_open... and it's
highly likely -EPROBE_DEFER will end up being returned to userspace
through either netlink or netdev ioctls.

Since EPROBE_DEFER is not an error number that we export to
userspace, this should basically never be exposed to userspace, yet
we have a path that it _could_ be exposed if the above condition
is true.

If device_pending_probe() returns true e.g. during initial boot up
while modules are being loaded - maybe the phy driver doesn't have
all the resources it needs because of some other module that hasn't
finished initialising - then we have a window where this will be
exposed to userspace.

So, do we need to fix all the network drivers to do something if
their .ndo_open method encounters this? If so, what? Sleep a bit
and try again? How many times to retry? Convert the error code into
something else, causing userspace to fail where it worked before? If
so which error code?

I think this needs to be thought through a bit better. In this case,
I feel that throwing -EPROBE_DEFER to solve one problem with one
subsystem can result in new problems elsewhere.

We did have an idea at one point about reserving some flag bits in
phydev->dev_flags for phylib use, but I don't think that happened.
If this is the direction we want to go, I think we need to have a
flag in dev_flags so that callers opt-in to the new behaviour whereas
callers such as from .ndo_open keep the old behaviour - because they
just aren't setup to handle an -EPROBE_DEFER return from these
functions.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!