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

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Message-ID: <20160624230609.GB1041@n2100.armlinux.org.uk>
Date:	Sat, 25 Jun 2016 00:06:09 +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 03:58:39PM -0700, Florian Fainelli wrote:
> On 06/24/2016 03:55 PM, Russell King - ARM Linux wrote:
> > 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?
> 
> In the sense that it is symetrical to the register code, probably.
> 
> > 
> > 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.
> >
> 
> What would you suggest we do instead? Would switching to IDA/IDR give us
> better results for instance (I have not looked too closely yet)?

I would expect an IDA to be suitable, because the IDA would track which
indexes (==addresses) are currently in-use.

If you want to go further, using an IDR would allow fixed_mdio_read() to
find the right fixed_phy struct without needing to loop over fmb->phys.
Whether that's worth it or not depends if you have a large number of
fixed phys.  I suspect we're talking about small quantities here though.

-- 
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