Re: [PATCH v2 4/7] drivers/base: Add interface framework

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, May 13, 2014 at 07:57:13PM +0200, Daniel Vetter wrote:
> On Tue, May 13, 2014 at 05:30:47PM +0200, Thierry Reding wrote:
> > From: Thierry Reding <treding@nvidia.com>
> > 
> > Some drivers, such as graphics drivers in the DRM subsystem, do not have
> > a real device that they can bind to. They are often composed of several
> > devices, each having their own driver. The master/component framework
> > can be used in these situations to collect the devices pertaining to one
> > logical device, wait until all of them have registered and then bind
> > them all at once.
> > 
> > For some situations this is only a partial solution. An implementation
> > of a master still needs to be registered with the system somehow. Many
> > drivers currently resort to creating a dummy device that a driver can
> > bind to and register the master against. This is problematic since it
> > requires (and presumes) knowledge about the system within drivers.
> > 
> > Furthermore there are setups where a suitable device already exists, but
> > is already bound to a driver. For example, on Tegra the following device
> > tree extract (simplified) represents the host1x device along with child
> > devices:
> > 
> > 	host1x {
> > 		display-controller {
> > 			...
> > 		};
> > 
> > 		display-controller {
> > 			...
> > 		};
> > 
> > 		hdmi {
> > 			...
> > 		};
> > 
> > 		dsi {
> > 			...
> > 		};
> > 
> > 		csi {
> > 			...
> > 		};
> > 
> > 		video-input {
> > 			...
> > 		};
> > 	};
> > 
> > Each of the child devices is in turn a client of host1x, in that it can
> > request resources (command stream DMA channels and syncpoints) from it.
> > To implement the DMA channel and syncpoint infrastructure, host1x comes
> > with its own driver. Children are implemented in separate drivers. In
> > Linux this set of devices would be exposed by DRM and V4L2 drivers.
> > 
> > However, neither the DRM nor the V4L2 drivers have a single device that
> > they can bind to. The DRM device is composed of the display controllers
> > and the various output devices, whereas the V4L2 device is composed of
> > one or more video input devices.
> > 
> > This patch introduces the concept of an interface and drivers that can
> > bind to a given interface. An interface can be exposed by any device,
> > and interface drivers can bind to these interfaces. Multiple drivers can
> > bind against a single interface. When a device is removed, interfaces
> > exposed by it will be removed as well, thereby removing the drivers that
> > were bound to those interfaces.
> > 
> > In the example above, the host1x device would expose the "tegra-host1x"
> > interface. DRM and V4L2 drivers can then bind to that interface and
> > instantiate the respective subsystem objects from there.
> > 
> > Signed-off-by: Thierry Reding <treding@nvidia.com>
> > ---
> > Note that I'd like to merge this through the Tegra DRM tree so that the
> > changes to the Tegra DRM driver later in this series can be merged at
> > the same time and are not delayed for another release cycle.
> > 
> > In particular that means that I'm looking for an Acked-by from Greg.
> > 
> >  drivers/base/Makefile     |   2 +-
> >  drivers/base/interface.c  | 186 ++++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/interface.h |  40 ++++++++++
> >  3 files changed, 227 insertions(+), 1 deletion(-)
> >  create mode 100644 drivers/base/interface.c
> >  create mode 100644 include/linux/interface.h
> 
> Hm, this interface stuff smells like bus drivers light. Should we instead
> have a pile of helpers to make creating new buses with match methods more
> trivial? There's a fairly big pile of small use-cases where this might be
> useful. In your case here all the host1x children would sit on a host1x
> bus. Admittedly I didn't look into the details.

I have no problem adding such "bus-light" functions, to make it easier
to create and implement a bus in the driver core, as I know it's really
heavy.  That's been on my "todo" list for over a decade now...

thanks,

greg k-h