netdev - Re: [PATCH] of: of_mdio: count number of regitered phys

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Message-ID: <6b022bcc-a670-da1d-5f5a-bdf4af667652@gmail.com>
Date:   Mon, 29 Jun 2020 20:35:11 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Andrew Lunn <andrew@...n.ch>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>
Cc:     hkallweit1@...il.com, linux@...linux.org.uk, robh+dt@...nel.org,
        frowand.list@...il.com, netdev@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] of: of_mdio: count number of regitered phys



On 6/29/2020 5:45 PM, Andrew Lunn wrote:
> On Mon, Jun 29, 2020 at 10:26:36AM +0300, Claudiu Beznea wrote:
>> In case of_mdiobus_register_phy()/of_mdiobus_register_device()
>> returns -ENODEV for all PHYs in device tree or for all scanned
>> PHYs there is a chance that of_mdiobus_register() to
>> return success code although no PHY devices were registered.
>> Add a counter that increments every time a PHY was registered
>> to avoid the above scenario.
> 
> Hi Claudiu
> 
> There is a danger here this will break something. Without this patch,
> an empty bus is O.K. But with this patch, a bus without a PHY is a
> problem.
> 
> Take for example FEC. It often comes in pairs. Each has an MDIO
> bus. But to save pins, there are some designs which place two PHYs on
> one bus, leaving the other empty. The driver unconditionally calls
> of_mdiobus_register() and if it returns an error, it will error out
> the probe. So i would not be too surprised if you get reports of
> missing interfaces with this patch.

Agreed, the potential for breakage here is too high especially given
this is fixing a hypothetical problem rather an an actual one. Even if
we were taking this from the angle of power management, runtime PM
should ensure that a MDIO bus with no slave, or no activity gets runtime
suspended.
-- 
Florian