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

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Date:   Tue, 14 Sep 2021 11:56:27 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Gerhard Engleder <gerhard@...leder-embedded.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net] Revert "net: phy: Uniform PHY driver access"



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@....com>

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