Re: [PATCH 2/5] drivercore: Add driver probe deferral mechanism

[David Daney]

On 10/14/2011 10:20 AM, Grant Likely wrote:
> On Fri, Oct 14, 2011 at 10:33 AM, Alan Stern<stern@xxxxxxxxxxxxxxxxxxx>  wrote:
> > On Fri, 14 Oct 2011, Grant Likely wrote:
> >
> > > > How can a device acquire children before it has a driver?
> > >
> > > There are a few potential situations in embedded systems (or at least
> > > nothing currently prevents it) where platform setup code constructs a
> > > device hierarchy without the aid of device drivers, and it is still
> > > possible for a parent device to be attached to a driver.  IIUC, SPARC
> > > creates an entire hierarchy of platform_devices from all the nodes in
> > > the OpenFirmware device tree, and any of those devices can be bound to
> > > a driver.  I don't like that approach, but at the very least it needs
> > > to be guarded against.
> >
> > Do these devices ever require deferred probes?
>
> Yes, they very well might.  However, I'm happy with the limitation
> that only leaf devices can take advantage of probe deferral.


I have:

I2C-Bus-A
  +--Mux-I2C (controlled by parent I2C-Bus-A)
      +---I2C-Bus-1
      |      +--GPIO-Expander-A
      |
      +---I2C-Bus-2
             +--GPIO-Expander-B

These all have a parent/child relationship so no deferral is needed, so 
far so good.


Then this:

MDIO-Bus-A
   +---Mux-MDIO (controlled by GPIO-Expander-A)
         +---MDIO-Bus-1
         |
         +---MDIO-Bus-2
               +---PHY-1
               |
               +---PHY-2

In this case the driver for Mux-MDIO needs to be deferred until *both* 
MDIO-Bus-A's driver *and* GPIO-Expander-B's driver are loaded.  A 
perfect use case for the patch.

Would you consider Mux-MDIO to be a 'leaf device'?  If not, then I have 
real problems with 'the limitation that only leaf devices can take 
advantage of probe deferral'

David Daney
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/