Re: [PATCH 1/5] pdev: Fix platform device resource linking

[Greg Kroah-Hartman]

On Tue, Aug 06, 2013 at 01:27:35PM +0300, Pantelis Antoniou wrote:
> Hi Greg,
> 
> On Aug 6, 2013, at 1:15 PM, Greg Kroah-Hartman wrote:
> 
> > On Tue, Aug 06, 2013 at 12:45:42PM +0300, Pantelis Antoniou wrote:
> >> Hi Greg,
> >> 
> >> On Aug 6, 2013, at 12:36 PM, Greg Kroah-Hartman wrote:
> >> 
> >>> On Tue, Aug 06, 2013 at 10:53:40AM +0300, Pantelis Antoniou wrote:
> >>>> Platform device removal uncovered a number of problems with
> >>>> the way resources are handled in the core platform code.
> >>>> 
> >>>> Resources now form child/parent linkages and this requires
> >>>> proper linking of the resources. On top of that the OF core
> >>>> directly creates it's own platform devices. Simplify things
> >>>> by providing helper functions that manage the linking properly.
> >>> 
> >>> Ugh, the OF core shouldn't be creating platform devices.  Well, yes, I
> >>> know it does that today, but ick, ick, ick.
> >>> 
> >> 
> >> Yep, ick, ick, ick is the correct form.
> >> 
> >>>> Two functions are provided:
> >>>> 
> >>>> platform_device_link_resources(), which links all the
> >>>> linkable resources (if not already linked).
> >>>> 
> >>>> and platform_device_unlink_resources(), which unlinks all the
> >>>> resources.
> >>> 
> >>> Why would anyone need to call this?  I'm getting the feeling that OF
> >>> should just have it's own bus of devices to handle this type of mess.
> >>> ACPI is going through the same rewrite for this same type of problem
> >>> (they did things differently.)  I suggest you work with the ACPI
> >>> developers to so the same thing they are, to solve it correctly for
> >>> everyone.
> >>> 
> >> 
> >> It's the same problem really. Another bus type might not fly well.
> >> The same device driver should be (in theory) be made to work unchanged
> >> either on an OF/ACPI/Fex( :) ) setup.
> > 
> > No, that's not quite true, a driver needs to know how to talk to the
> > bus, as that is how it communicates to the hardware.  It can be done for
> > different types of busses (see the OHCI USB controller for one example
> > of this), but a driver will have to know what type of bus it is on in
> > order to work properly.
> > 
> 
> In the case of OF & ACPI there is no 'bus'. The device is probably integrated
> in the SoC's silicon, but there is absolutely no way to 'probe' for it's existence;
> you have to use a-priori knowledge of the SoC's topology in order to configure it
> (along with any per-board specific information if there is any kind of shared 
> resource configuration - i.e. pinmuxing or something else).

Not all busses need to have the aiblity to "probe" for new devices,
that's not a requirement at all.  Some of them just "know" where the
devices are at in the driver model, and create the devices for the bus
just fine.

So don't think that just because of that lack of probing, they should be
on the "platform" bus at all.  Platform is for the "oh crap, I have no
way to bind this to anything else, make it a platform device then."

> There are the 3 well known methods to do so in the Linux kernel right now:
> 
> 1) Board files in which the configuration information is stored in the per-board
> platform file encoded in platform data structures.
> 
> 2) OF, in which case the information is provided via the flat device tree blob
> the bootloader provides.
> 
> 3) ACPI in which case the information is provided via the firmware's ACPI tables
> (I'm not overly familiar with ACPI, so there might be some more nuance here).
> 
> The device driver for all these cases is absolutely the same; the only place where
> it differs it's in the way it uses to retrieve that configuration information.

Not at all, they all should be differing in how to talk to the hardware,
right?  Or even if not, it should be pretty trivial for all of them to
have drivers that bind to a multiple of different types of busses just
fine, no need to want to abuse the platform code because people feel
lazy.

> > What specifically would you move into there?
> > 
> 
> Pretty much everything that's in the union of platform_device & whatever 
> acpi uses to hold it's configuration info.

Specifically what is that?  I've never had a problem with moving stuff
into struct device that is common among different subsystems that can
share the pointers, as that is exactly what the structure is there for.

thanks,

greg k-h
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