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

Matt Porter
2013-08-07T17:13:55Z

[trimmed my old email]

On Wed, Aug 07, 2013 at 10:37:17AM +0300, Pantelis Antoniou wrote:
> Hi Greg,
>
> On Aug 7, 2013, at 8:56 AM, Greg Kroah-Hartman wrote:
>
> > 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."
> >
>
> I'm not sure if you remember, but a long time ago when OF started getting
> into the kernel, there actually was an OCP (On Chip Peripheral) bus,
> and the switch to platform devices was mandated by kernel people, by their
> insistance that platform devices would cover every case.
>
> See here:
>
> http://lkml.indiana.edu/hypermail/linux/kernel/0405.1/0930.html
>
> I'm sure Matt Porter can shed some light on the exchange that led to the
> abandonment of the ocp bus concept.

Heh, that OCP support looks a bit antiquated by today's standards. If it
helps, http://lkml.indiana.edu/hypermail/linux/kernel/0501.2/0696.html
is the posting where Kumar starts taking arch/ppc away from using
drivers/ocp/. I can't find any public discussion that led to this, but I
recall the common advice was "just use platform devices". This was,
incidentally, just before the move to arch/powerpc (and DT for all)
began.

Keep in mind that this is 8ish years ago before embedded was
fashionable since we didn't all have Linux machines in our pockets.
I suspect that advice was given because nobody cared about
platform_device removal and it worked for the use cases at the time.

-Matt