Re: [PATCH 2/3] spi / ACPI: add ACPI enumeration support

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Saturday, November 10, 2012 10:14:47 AM Bjorn Helgaas wrote:
> On Sat, Nov 10, 2012 at 4:10 AM, Rafael J. Wysocki <rjw@sisk.pl> wrote:
> > On Friday, November 09, 2012 09:53:26 AM Bjorn Helgaas wrote:
> >> On Fri, Nov 9, 2012 at 9:43 AM, Grant Likely <grant.likely@secretlab.ca> wrote:
> >> > On Fri, Nov 9, 2012 at 4:35 PM, Bjorn Helgaas <bhelgaas@google.com> wrote:
> >> >> On Fri, Nov 9, 2012 at 8:45 AM, Grant Likely <grant.likely@secretlab.ca> wrote:
> >> >>> On Fri, Nov 9, 2012 at 3:11 PM, Bjorn Helgaas <bhelgaas@google.com> wrote:
> >> >>>> [+cc Greg, Peter, Tony since they acked the original patch [1]]
> >> >>>>
> >> >>>> On Thu, Nov 8, 2012 at 1:04 PM, Mika Westerberg
> >> >>>> <mika.westerberg@linux.intel.com> wrote:
> >> >>>>> On Thu, Nov 08, 2012 at 12:32:25PM -0700, Bjorn Helgaas wrote:
> >> >>>>>> Struct device_driver is a generic structure, so it seems strange to
> >> >>>>>> have to include non-generic things like of_device_id and now
> >> >>>>>> acpi_match_table there.
> >> >>>>>
> >> >>>>> Yes, but in a sense the DT and ACPI are "generic". So that they are used to
> >> >>>>> describe the configuration of a machine.
> >> >>>>
> >> >>>> What I meant by "generic" was "useful across all architectures."  The
> >> >>>> new acpi_match_table and acpi_handle fields [1] are not generic in
> >> >>>> that sense because they're present on all architectures but used only
> >> >>>> on x86 and ia64.  The existing of_match_table and of_node are
> >> >>>> similarly unused on many architectures.  This doesn't seem like a
> >> >>>> scalable strategy to me.  Are we going to add a pnpbios_node for x86
> >> >>>> PNPBIOS machines without ACPI, a pdc_hpa for parisc machines with PDC,
> >> >>>> etc.?
> >> >>>>
> >> >>>> [1] https://patchwork.kernel.org/patch/1677221/
> >> >>>
> >> >>> Ultimately yes, I think that is what we want to do,
> >> >>
> >> >> Just to be clear, you think we *should* add things like pnpbios_node,
> >> >> pdc_hpa, etc., to struct device, one field for every scheme of telling
> >> >> the OS about non-enumerable devices, where only one of the N fields is
> >> >> used on any given machine?  That seems surprising to me, but maybe I
> >> >> just need to be educated :)
> >> >
> >> > Ah, I see what you're asking.
> >> >
> >> > In the short term, yes but only because we don't have any other
> >> > alternative. What I'd really rather have is a safe way to attach datum
> >> > (ie. acpi_device or device_node) to a struct device and get it back
> >> > later in a type safe way.
> >>
> >> Yep, *that* makes perfect sense to me.  Something along these lines, maybe:
> >>
> >>     #define dev_is_acpi(d)    ((d)->bus == &acpi_bus_type)
> >
> > No, that's not right.  It won't work for things like SPI and I2C with a
> > "backing" ACPI device node anyway (and for PCI too, by the way :-)).
> >
> >>     #define dev_acpi_handle(d)    (dev_is_acpi(d) ? (acpi_handle)
> >> d->datum : NULL)
> >
> > The problem basically is how we can tell that the given struct device has
> > a "backing" object containing device information (e.g. resources) and what
> > that "backing" object is.  For device trees that would be a struct device_node
> > and for ACPI that would be an acpi_handle or a struct acpi_device etc.  And by
> > the way, they _can_ be used simultaneously, in principle.
> >
> > So we need something like of_node(dev) or acpi_node(dev), but that can't be
> > something following two pointers or calling a function just in order to check
> > if the pointer _is_ _there_ in either case.
> >
> > And since we added of_node to struct device at one point, it is only logical to
> > treat ACPI in the same way.  If we come up with a better idea _later_, then we
> > can convert _all_ things to this new idea, whatever it is.
> >
> > Are you seriously expecting us to come up with such an idea on the fly just so
> > that we can use ACPI support, which already is there in the form of
> > archdata.acpi_handle anyway, on equal footing with Device Trees?
> 
> Of course not.  I'm just trying to understand where we're headed.
> That was not obvious from the patches I've seen so far.

No, it wasn't, fair enough.

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.