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

[Felipe Balbi]

Hi,

On Mon, Aug 01, 2011 at 04:44:20PM +0100, Grant Likely wrote:
> 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.

considering that struct omap_device isn't ARM-wide, is it really wise to
to do that ? I guess a void * will do better here.

-- 
balbi
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