[RFC/PATCH 7/7] WIP: HACK/RFC: omap_device: begin to decouple platform_device from omap_device

[Grant Likely]

On Mon, Aug 1, 2011 at 4:42 PM, Kevin Hilman <khilman at ti.com> wrote:
> Russell King - ARM Linux <linux at arm.linux.org.uk> writes:
>
>> On Sat, Jul 30, 2011 at 08:58:07PM -0600, Grant Likely wrote:
>>> On Sat, Jul 30, 2011 at 01:03:32PM +0100, Russell King - ARM Linux wrote:
>>> > On Thu, Jul 21, 2011 at 04:52:18PM -0700, Kevin Hilman wrote:
>>> > > Rather than embedding a struct platform_device inside a struct
>>> > > omap_device, decouple them, leaving only a pointer to the
>>> > > platform_device inside the omap_device.
>>> > >
>>> > > This patch uses devres to allocate and attach the omap_device to the
>>> > > struct device, so finding an omap_device from a struct device means
>>> > > using devres_find(), and the to_omap_device() helper function was
>>> > > modified accordingly.
>>> > >
>>> > > RFC/Hack alert:
>>> > >
>>> > > Currently the driver core (drivers/base/dd.c) doesn't expect any
>>> > > devres resources to exist before the driver's ->probe() is called.  In
>>> > > this patch, I just comment out the warning, but we'll need to
>>> > > understand why that limitation exists, and if it's a real limitation.
>>> > > A first glance suggests that it's not really needed.  If this is a
>>> > > true limitation, we'll need to find some way other than devres to
>>> > > attach an omap_device to a struct device.
>>> > >
>>> > > On OMAP, we will need an omap_device attached to a struct device
>>> > > before probe because device HW may be disabled in probe and drivers
>>> > > are expected to use runtime PM in ->probe() to activate the hardware
>>> > > before access.  Because the runtime PM API calls use omap_device (via
>>> > > our PM domain layer), we need omap_device attached to a
>>> > > platform_device before probe.
>>> >
>>> > This feels really wrong to overload devres with this.  devres purpose is
>>> > to provide the device's _drivers_ with a way to allocate and free resources
>>> > in such a way to avoid leaks on .remove or probe failure.  So I think that
>>> > overloading it with something that has a different lifetime is completely
>>> > wrong.
>>>
>>> I disagree; extending devres to also handle device lifetime scoped
>>> resources makes perfect sense. It is a clean extension, and struct device
>>> is really getting rather huge.  I certainly prefer re-scoping devres
>>> to adding more elements to struct device.
>>
>> The point is you're asking something which is designed to have a well
>> defined lifetime of driver-bind...driver-unbind to do a lot more than
>> that - extending its lifetime conditional on some flag to the entire
>> device lifetime instead.
>>
>> Such extensions tend to be a disaster - and a recipe for confusion as
>> people will no longer have a clear idea of the lifetime issues.  We
>> already know that people absolutely struggle to understand lifed
>> objects - we see it time and time again where people directly kfree
>> stuff like platform devices without thinking about whether they're
>> still in use.
>>
>> So no, extending devres is a _big_ mistake and is far from clean.
>>
>> Not only that, but if most of the platform devices are omap devices,
>> (as it would be on OMAP), you'll consume more memory through having to
>> have the additional management structs for the omap_device stuff than
>> a simple pointer.
>>
>> As for struct device getting rather huge, last time I looked it contained
>> a load of stuff which was there whether or not a platform used it.  Eg,
>> you get 4 bytes wasted for an of_node whether you have DT support or not.
>> If sizeof(struct device) really is a problem, then it needs the unused
>> stuff ifdef'd away.  So I don't buy the "its getting rather huge" argument.
>>
>>> > We could add a new member to struct dev_archdata or pdev_archdata to carry
>>> > a pointer to this data, which I think would be a far cleaner (and saner)
>>> > way to deal with this.  In much the same way as we already have an of_node
>>> > member in struct device.
>>>
>>> Since this is just for omap_device, which by definition is arm-only, I
>>> don't have any strong objection to using pdev_archdata. If it was
>>> cross-architecture, then I'd argue strongly for the devres approach.
>>
>> Indeed, which is why I think putting it in the platform device archdata
>> makes total sense, more sense than buggering up the clean devres lifetime
>> rules that we have today.
>
> OK, so I'll continue this approach using pdev_archdata, which would work
> fine for me.  It's currently empty, so I'll just add a 'void *' if it's
> OK with you Russell:
>
> diff --git a/arch/arm/include/asm/device.h b/arch/arm/include/asm/device.h
> index 9f390ce..bb777cd 100644
> --- a/arch/arm/include/asm/device.h
> +++ b/arch/arm/include/asm/device.h
> @@ -13,6 +13,7 @@ struct dev_archdata {
>  };
>
>  struct pdev_archdata {
> +       void *p;
>  };

struct omap_device *p;

Otherwise it is just asking for type safety problems.

g.