On Fri, Mar 21, 2025 at 10:04:26AM +0100, Danilo Krummrich wrote: > On Thu, Mar 20, 2025 at 06:40:56PM -0700, Greg KH wrote: > > On Thu, Mar 20, 2025 at 11:27:43PM +0100, Danilo Krummrich wrote: > > > Device::parent() returns a reference to the device' parent device, if > > > any. > > > > Ok, but why? You don't use it in this series, or did I miss it? > > Indeed, it should rather be at the auxbus series. > > > A driver shouldn't care about the parent of a device, as it shouldn't > > really know what it is. So what is this needed for? > > Generally, that's true. I use in the auxbus example and in nova-drm [1] (which > is connected through the auxbus to nova-core) to gather some information about > the device managed by nova-core. > > Later on, since that's surely not enough, we'll have an interface to nova-core > that takes the corresponding auxiliary device. nova-core can then check whether > the given device originates from nova-core, and through the parent find its own > device. > > [1] https://gitlab.freedesktop.org/drm/nova/-/blob/staging/nova-drm/drivers/gpu/drm/nova/driver.rs Another category of drivers that came to my mind and seems valid for this is MFD. Other than that I found a couple of cases where platform drivers interact with their corresponding parent devices (mostly embedded platforms where the topology is known), as well as a couple of HID devices that access their parent to issue USB transactions etc., which all seems more like an abuse due to a lack of proper APIs, which may or may not exist at the time the corresponding driver was written. So, maybe we should make Device::parent() crate private instead, such that it can't be accessed by drivers, but only the core abstractions and instead only provide accessors for the parent device for specific bus devices, where this is reasonable to be used by drivers, e.g. auxiliary.
