Re: [PATCH 2/2] of_mdio: Allow the DT to specify the phy ID and avoid autoprobing

From: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
To: Florian Fainelli <f.fainelli@gmail.com>
Cc: Rob Herring <robh+dt@kernel.org>,
	Grant Likely <grant.likely@linaro.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	netdev <netdev@vger.kernel.org>
Subject: Re: [PATCH 2/2] of_mdio: Allow the DT to specify the phy ID and avoid autoprobing
Date: Fri, 31 Jan 2014 15:55:13 -0700	[thread overview]
Message-ID: <20140131225513.GA2519@obsidianresearch.com> (raw)
In-Reply-To: <CAGVrzcbVo-Zzm9kLhiptf-qWgPH9XSs29mW8g2=g=Bq=PCb0_w@mail.gmail.com>

On Fri, Jan 31, 2014 at 02:24:52PM -0800, Florian Fainelli wrote:

> > This is necessary to support phy's that cannot be autoprobed when
> > of_mdiobus_register is called. Specifically, my case has the phy in reset at
> > of_mdiobus_register, the reset is only released once the ethernet driver
> > starts, before it attaches to the phy.
> 
> Who is responsible for bringing the PHY out of reset, is it the
> Ethernet/MDIO bus driver? Is the PHY put into reset using, e.g a GPIO
> line or any sort of reset controller, if so, should not we have some
> sort of reset handle node and handle that in a generic manner?

The Phy Reset is connected to a GPIO, and the ethernet driver has code
to switch the GPIO out of reset. The phy is kept in reset until the
ethernet device is opened, and Linux is booted with the phy reset
asserted.

> Is your DTS or DTB hardcoding the PHY id, or are you having your
> bootloader detect the exact PHY for you, then putting back the PHY
> into reset to save power, until someone uses that PHY again?

For our uses the Phy ID is hardcoded. There is only a single part that
will fit on the board. So the bootloader doesn't touch the phy. If
there were alternate parts we'd get the part kind from the EEPROM that
stores the MAC address/etc.

> > +       while (cplen > 0) {
> > +               if (sscanf(cp, "ethernet-phy-id%4x.%4x", &upper, &lower) == 2) {
> 
> You might want to guard against 0x0 and 0xffff just in case whoever
> fills this information in the Device Tree was reading bogus data out
> of the MDIO bus, otherwise, chances are that the "Generic PHY" driver
> will be picked up, and it might still not be appropriate for driving
> your PHY chip.

Having the bootloader read the phy ID just to fill in this compatible
string isn't really the point. In every normal case I think it makes
sense to let Linux autoprobe the phy id. The use for this compatible
string is to defeat the autoprobe for situations where it is not
appropriate.

Thanks,
Jason