netdev - Re: [PATCH] net: phy: leds: Trigger leds only if PHY speed is known

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Date:   Wed, 11 Aug 2021 16:10:45 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     "Ivan T. Ivanov" <iivanov@...e.de>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: phy: leds: Trigger leds only if PHY speed is known

On Wed, Aug 11, 2021 at 12:51:04PM +0300, Ivan T. Ivanov wrote:
> There are a few callers of this, but then we have a few callers of
> genphy_read_status() too, which are outside just implementing 
> phy_driver->read_status() and don't use locking.

I think we need to strongly discourage people using the genphy*
functions directly from anything except phylib driver code. Any
such use is a layering violation.

I think it does make sense to have a set of lower-level API
functions that do the locking necessary, rather than having the
phylib locking spread out across multiple network drivers. It's
better to have it in one place.

> Then there a few users of phy_init_eee() which uses phy_read_status(),
> again without locking.

That is kind-of a special case - phy_init_eee() can be called by
network drivers from within the phylib adjust_link() callback, and
this callback is made while holding the phydev's lock. So those
cases are safe.

If phy_init_eee() is called outside of that, and the lock isn't
taken, then yes, it's buggy.

This all said, I can't say that I have particularly liked the
phy_init_eee() API. It seems to mix interface-up like configuration
(do we wish clocks to stop in EEE) with working out whether EEE
should be enabled for the speed/duplex that we've just read from
the PHY. However, most users of this function are being called as a
result of a link-up event when we already know the link parameters,
so we shouldn't be re-interrogating the PHY at this point. It seems
to me to be entirely unnecessary.

> I think will be easier if we protect public phylib API internally with
> lock, otherwise it is easy to make mistakes, obviously. But still this
> will not protect users which directly dereference phy_device members.

As Andrew says... but there are some members that network drivers
have to access due to the design of phylib, such as speed, duplex,
*pause, link, etc. Indeed these can change at any time when
phydev->lock is not held due to the action of phylib polling or
link interrupts from the PHY. So, accessing them outside of the
adjust_link() callback without holding the lock is racy.

Note that phylink's usage is similarly safe to the adjust_link()
callback; its access to these members is similarly protected by
phydev->lock taken in the phylib state machine - we use the
slightly lower-level phy_link_change() hook which avoids some of
the help that phylib provides to network drivers (which phylink
really doesn't want phylib managing.)

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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