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

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Message-ID: <24155036.h7GFQbIk2D@vostro.rjw.lan>
Date:	Sat, 10 Nov 2012 12:10:27 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Grant Likely <grant.likely@...retlab.ca>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	linux-kernel@...r.kernel.org, lenb@...nel.org,
	rafael.j.wysocki@...el.com, broonie@...nsource.wolfsonmicro.com,
	linus.walleij@...aro.org, khali@...ux-fr.org, ben-linux@...ff.org,
	w.sang@...gutronix.de, mathias.nyman@...ux.intel.com,
	linux-acpi@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"H. Peter Anvin" <hpa@...or.com>, Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH 2/3] spi / ACPI: add ACPI enumeration support

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@...retlab.ca> wrote:
> > On Fri, Nov 9, 2012 at 4:35 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> >> On Fri, Nov 9, 2012 at 8:45 AM, Grant Likely <grant.likely@...retlab.ca> wrote:
> >>> On Fri, Nov 9, 2012 at 3:11 PM, Bjorn Helgaas <bhelgaas@...gle.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@...ux.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?

Rafael


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