On Wed, Feb 10, 2021 at 02:13:48PM -0800, Saravana Kannan wrote: > Hi, > > This email was triggered by this other email[1]. > > Why is phy_attach_direct() directly calling device_bind_driver() > instead of using bus_probe_device()?
Hi Saravana So this is to do with the generic PHY, which is a special case. First the normal case. The MDIO bus driver registers an MDIO bus using mdiobus_register(). This will enumerate the bus, finding PHYs on it. Each PHY device is registered with the device core, using the usual device_add(). The core will go through the registered PHY drivers and see if one can drive this hardware, based on the ID registers the PHY has at address 2 and 3. If a match is found, the driver probes the device, all in the usual way. Sometime later, the MAC driver wants to make use of the PHY device. This is often in the open() call of the MAC driver, when the interface is configured up. The MAC driver asks phylib to associate a PHY devices to the MAC device. In the normal case, the PHY has been probed, and everything is good to go. However, sometimes, there is no driver for the PHY. There is no driver for that hardware. Or the driver has not been built, or it is not on the disk, etc. So the device core has not been able to probe it. However, IEEE 802.3 clause 22 defines a minimum set of registers a PHY should support. And most PHY devices have this minimum. So there is a fall back driver, the generic PHY driver. It assumes the minimum registers are available, and does its best to drive the hardware. It often works, but not always. So if the MAC asks phylib to connect to a PHY which does not have a driver, we forcefully bind the generic driver to the device, and hope for the best. We don't actually recommend using the generic driver. Use the specific driver for the hardware. But the generic driver can at least get you going, allow you to scp the correct driver onto the system, etc. Andrew
