netdev - Re: [PATCH net-next 1/3] net: phy: Support enabling clocks prior to bus probe

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Message-ID: <20200903212549.GE3112546@lunn.ch>
Date:   Thu, 3 Sep 2020 23:25:49 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     netdev@...r.kernel.org, adam.rudzinski@....net.pl,
        m.felsch@...gutronix.de, hkallweit1@...il.com,
        richard.leitner@...data.com, zhengdejin5@...il.com,
        devicetree@...r.kernel.org, kernel@...gutronix.de, kuba@...nel.org,
        robh+dt@...nel.org
Subject: Re: [PATCH net-next 1/3] net: phy: Support enabling clocks prior to
 bus probe

On Wed, Sep 02, 2020 at 09:39:45PM -0700, Florian Fainelli wrote:
> Some Ethernet PHYs may require that their clock, which typically drives
> their logic to respond to reads on the MDIO bus be enabled before
> issusing a MDIO bus scan.
> 
> We have a chicken and egg problem though which is that we cannot enable
> a given Ethernet PHY's device clock until we have a phy_device instance
> create and called the driver's probe function. This will not happen
> unless we are successful in probing the PHY device, which requires its
> clock(s) to be turned on.
> 
> For DT based systems we can solve this by using of_clk_get() which
> operates on a device_node reference, and make sure that all clocks
> associaed with the node are enabled prior to doing any reads towards the
> device. In order to avoid drivers having to know the a priori reference
> count of the resources, we drop them back to 0 right before calling
> ->probe() which is then supposed to manage the resources normally.

Hi Florian

This seems asymmetric. The resources are enabled in
of_mdiobus_phy_device_register() but disabled in phy_probe().

What we are really interested in is having the clock ticking while
trying to read the ID registers. Could the enable/disable be placed
around the call to get_phy_id()?

       Andrew