Re: [PATCH v2 net] Revert "net: phy: Uniform PHY driver access"

From: Florian Fainelli
Date: Tue Sep 14 2021 - 14:56:48 EST




On 9/14/2021 7:05 AM, Vladimir Oltean wrote:
This reverts commit 3ac8eed62596387214869319379c1fcba264d8c6, which did
more than it said on the box, and not only it replaced to_phy_driver
with phydev->drv, but it also removed the "!drv" check, without actually
explaining why that is fine.

That patch in fact breaks suspend/resume on any system which has PHY
devices with no drivers bound.

The stack trace is:

Unable to handle kernel NULL pointer dereference at virtual address 00000000000000e8
pc : mdio_bus_phy_suspend+0xd8/0xec
lr : dpm_run_callback+0x38/0x90
Call trace:
mdio_bus_phy_suspend+0xd8/0xec
dpm_run_callback+0x38/0x90
__device_suspend+0x108/0x3cc
dpm_suspend+0x140/0x210
dpm_suspend_start+0x7c/0xa0
suspend_devices_and_enter+0x13c/0x540
pm_suspend+0x2a4/0x330

Examples why that assumption is not fine:

- There is an MDIO bus with a PHY device that doesn't have a specific
PHY driver loaded, because mdiobus_register() automatically creates a
PHY device for it but there is no specific PHY driver in the system.
Normally under those circumstances, the generic PHY driver will be
bound lazily to it (at phy_attach_direct time). But some Ethernet
drivers attach to their PHY at .ndo_open time. Until then it, the
to-be-driven-by-genphy PHY device will not have a driver. The blamed
patch amounts to saying "you need to open all net devices before the
system can suspend, to avoid the NULL pointer dereference".

- There is any raw MDIO device which has 'plausible' values in the PHY
ID registers 2 and 3, which is located on an MDIO bus whose driver
does not set bus->phy_mask = ~0 (which prevents auto-scanning of PHY
devices). An example could be a MAC's internal MDIO bus with PCS
devices on it, for serial links such as SGMII. PHY devices will get
created for those PCSes too, due to that MDIO bus auto-scanning, and
although those PHY devices are not used, they do not bother anybody
either. PCS devices are usually managed in Linux as raw MDIO devices.
Nonetheless, they do not have a PHY driver, nor does anybody attempt
to connect to them (because they are not a PHY), and therefore this
patch breaks that.

The goal itself of the patch is questionable, so I am going for a
straight revert. to_phy_driver does not seem to have a need to be
replaced by phydev->drv, in fact that might even trigger code paths
which were not given too deep of a thought.

For instance:

phy_probe populates phydev->drv at the beginning, but does not clean it
up on any error (including EPROBE_DEFER). So if the phydev driver
requests probe deferral, phydev->drv will remain populated despite there
being no driver bound.

If a system suspend starts in between the initial probe deferral request
and the subsequent probe retry, we will be calling the phydev->drv->suspend
method, but _before_ any phydev->drv->probe call has succeeded.

That is to say, if the phydev->drv is allocating any driver-private data
structure in ->probe, it pretty much expects that data structure to be
available in ->suspend. But it may not. That is a pretty insane
environment to present to PHY drivers.

In the code structure before the blamed patch, mdio_bus_phy_may_suspend
would just say "no, don't suspend" to any PHY device which does not have
a driver pointer _in_the_device_structure_ (not the phydev->drv). That
would essentially ensure that ->suspend will never get called for a
device that has not yet successfully completed probe. This is the code
structure the patch is returning to, via the revert.

Fixes: 3ac8eed62596 ("net: phy: Uniform PHY driver access")
Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>

Acked-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
--
Florian