Re: [PATCH net] net: phy: Decrement phy_fixed_addr during unregister

[Florian Fainelli <f.fainelli@gmail.com>]

On 06/24/2016 04:06 PM, Russell King - ARM Linux wrote:
> 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.

OK, thanks!

> 
> 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.

Since I am targetting this as a bugfix, the switch to IDA seems more
appropriate to be backported, but yes, that's a good idea though.

> 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.
> 

Yes, at the moment we are limited to 32 PHYs maximum, just like a real
MDIO bus, which in some systems could actually be not enough, but then
you run into other problems, like the need to register more than a
single fixed MDIO bus driver to get a larger address space...
-- 
Florian