Message-ID: <545A8E62.7000004@linux.intel.com>
Date: Wed, 05 Nov 2014 12:53:54 -0800
From: Darren Hart <dvhart@linux.intel.com>
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To: "Rafael J. Wysocki" <rjw@rjwysocki.net>
CC: Grant Likely <grant.likely@secretlab.ca>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        Alexandre Courbot <acourbot@nvidia.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>,
        ACPI Devel Mailing List <linux-acpi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] ACPI / GPIO: Pass index to acpi_get_gpiod_by_index()
 when using properties
References: <1414494927-204923-1-git-send-email-mika.westerberg@linux.intel.com> <3196606.Sy6xFg88zg@vostro.rjw.lan> <5459646E.9000707@linux.intel.com> <2251117.W1kfIOl2SD@vostro.rjw.lan>
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On 11/5/14 12:59, Rafael J. Wysocki wrote:
> On Tuesday, November 04, 2014 03:42:38 PM Darren Hart wrote:
>>
>> On 11/4/14 14:54, Rafael J. Wysocki wrote:
>>
>>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>>> Subject: ACPI / property: Drop size_prop from acpi_dev_get_property_reference()
>>>
>>> The size_prop argument of the recently added function
>>> acpi_dev_get_property_reference() is not used by the only current
>>> caller of that function and is very unlikely to be used at any time
>>> going forward.
>>>
>>> Namely, for a property whose value is a list of items each containing
>>> a references to a device object possibly accompanied by some integers,
>>> the number of items in the list can always be computed as the number
>>> of elements of type ACPI_TYPE_LOCAL_REFERENCE in the property package.
>>> Thus it should never be necessary to provide an additional "cells"
>>> property with a value equal to the number of items in that list.
>>
>> In this case, do we never expect a property to contain more than one
>> ACPI_TYPE_LOCAL_REFERENCE?
>>
>> Package () { "foobar",
>>               Package () {
>>                   "PCI0.FOO", "PCI0.BAR", 0, 1, 0,
>>                   "PCI0.FOO", "PCI0.BAR2", 0, 1, 1
>>               }
>> }
>>
>> This seems like it could be useful for connecting various types of
>> devices together, but I confess not to have a specific exmaple in mind.
>> It does concern me to limit the data format in this way.
> 
> We don't support this even with size_prop, so it doesn't seem to be relevant here.
> 
> Now, if we were to support this, I'd rather not use acpi_dev_get_property_reference()
> for that, but add a new function specifically for it.  Moreover, I would extend the
> format definition then so that we could do
> 
> Package () {
> 	"foobar", Package () {
> 			Package () {"PCI0.FOO", "PCI0.BAR", 0, 1, 0},
> 			Package () {"PCI0.FOO", "PCI0.BAR2", 0, 1, 1}
> 	}
> }
> 
> in which case adding a special "size" property could be avoided.
> 
> That said, I have no idea why it might be necessary.  One reference in a property
> value means that we're connecting the current node (the owner of the _DSD
> containing that property) with some other node in the namespace.  Two references
> in there would mean that the current node is to be connected with *two* other
> nodes in the namespace at the same time.  That raises some questions that I'd
> rather not consider in detail here, unless you insist. ;-)
> 
>> I suppose should such a case become necessary, we can deal with the
>> issue then - and still avoid having a potential abuse point in the API
>> from the start.
> 
> What we have today is sufficient for all of the cases we've considered so far.
> If we find a case where it is not sufficient, we'll need to consider extending
> the data format as well as the API.
> 
> Rafael

Agreed on all points. Thanks Rafael.

-- 
Darren Hart
Intel Open Source Technology Center
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