netdev - Re: [PATCH net] net: phy: Decrement phy_fixed

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Message-ID: <20160624225527.GA1041@n2100.armlinux.org.uk>
Date:	Fri, 24 Jun 2016 23:55:27 +0100
From:	Russell King - ARM Linux <linux@...linux.org.uk>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, andrew@...n.ch,
	thomas.petazzoni@...e-electrons.com
Subject: Re: [PATCH net] net: phy: Decrement phy_fixed_addr during unregister

On Fri, Jun 24, 2016 at 03:44:11PM -0700, Florian Fainelli wrote:
> If we have a system which uses fixed PHY devices and calls
> fixed_phy_register() then fixed_phy_unregister() we can exhaust the
> number of fixed PHYs available after a while, since we keep incrementing
> the variable phy_fixed_addr, but we never decrement it.
> 
> This patch fixes that by decrementing phy_fixed_addr during
> fixed_phy_del(), and in order to do that, we need to move the
> phy_fixed_addr integer and its spinlock above that function.

Is this really a good idea?

What if we have two fixed phys register, and the first one is
unregistered and a new one subsequently registered?

First phy registered, gets address 0, phy_fixed_addr becomes 1.
Second phy registered, gets address 1, phy_fixed_addr becomes 2.
First phy is unregistered, phy_fixed_addr becomes 1.
Third phy registered, gets address 1, conflicts with the second phy.

Obviously not a good outcome.

-- 
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