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

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Date:	Fri, 14 Oct 2011 13:03:27 -0600
From:	Grant Likely <grant.likely@...retlab.ca>
To:	David Daney <ddaney.cavm@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Ming Lei <tom.leiming@...il.com>,
	Andrei Warkentin <awarkentin@...are.com>,
	Greg KH <greg@...ah.com>, Dilan Lee <dilee@...dia.com>,
	"G, Manjunath Kondaiah" <manjugk@...com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Josh Triplett <josh@...htriplett.org>, Manjunath@...per.es,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Linux PM List <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [PATCH 2/5] drivercore: Add driver probe deferral mechanism

On Fri, Oct 14, 2011 at 12:56 PM, David Daney <ddaney.cavm@...il.com> wrote:
> On 10/14/2011 10:20 AM, Grant Likely wrote:
>>
>> On Fri, Oct 14, 2011 at 10:33 AM, Alan Stern<stern@...land.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.

g.
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