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

David Daney
2011-10-14T19:09:04Z

On 10/14/2011 12:03 PM, Grant Likely wrote:
> On Fri, Oct 14, 2011 at 12:56 PM, David Daney<ddaney.cavm@gmail.com> wrote:
>> On 10/14/2011 10:20 AM, Grant Likely wrote:
>>>
>>> On Fri, Oct 14, 2011 at 10:33 AM, Alan Stern<stern@rowland.harvard.edu>
>>> 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'
>
> leaf device **at the time of its driver probe**. :-) After the
> device has all of its dependencies met, it can freely add child
> devices. In your case, the child devices will get added by the
> Mux-MDIO device driver, so all is good.
>

Indeed. Thanks for the confirmation.

David Daney