On Wed, Sep 02, 2015 at 04:43:38PM -0700, Doug Anderson wrote: > Russell, > > On Wed, Sep 2, 2015 at 3:50 PM, Russell King - ARM Linux > <linux@xxxxxxxxxxxxxxxx> wrote: > > Never copy the of_node from one device to another. That allows the > > bus matching to unintentionally match the of_node against the wrong > > driver. > > Can you be more specific about what problems you'd expect. It seems > like a terribly common practice to do this, but maybe I'm > misunderstanding: The problem with copying an of_node from one struct device to another struct device is of_match_device(). Devices in a DT setup are bound to drivers using dev->of_node - when a new device or driver is registered onto a bus, the new object is matched against all un-bound objects (devices or drivers as appropriate) comparing the compatible string gained from dev->of_node against the list of of_device_id strings provided by the driver. When a match is found, the two are attempted to be bound together. Consider what happens when you have a platform device with an of_node, and you bind that to a driver matched via the compatible string. The driver creates a new platform device, and copies the of_node to this new device so the new device can have access to the properties of the of_node. What happens next? Does the new device bind to the same driver (and thus we enter an infinite loop) or does it bind to some other driver as the author originally hoped? Depends whether the other driver is earlier in the list of drivers or later, whether it's in the kernel or not. It's less of a problem when you copy the of_node between two different buses, because they won't match the same driver, but this is a dangerous act - if stuff copies it in one direction, what's to stop someone copying it back to a new device on the original bus. As soon as that happens, things can go awol. It would be a good idea if either: 1) this never happened or 2) we had a flag which said "ignore the of_node for driver matching" And yes, we do have drivers which do exactly what I said above. They work through luck rather than design. -- FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx http://lists.freedesktop.org/mailman/listinfo/dri-devel
