Re: [PATCH v2 01/12] ACPI/IORT: Make iort_match_node_callback walk the ACPI namespace for NC

From: Robin Murphy <robin.murphy@arm.com>
To: Hanjun Guo <guohanjun@huawei.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: devicetree@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	linux-pci@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Diana Craciun <diana.craciun@oss.nxp.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Makarand Pawagi <makarand.pawagi@nxp.com>,
	linux-acpi@vger.kernel.org, iommu@lists.linux-foundation.org,
	Rob Herring <robh+dt@kernel.org>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Will Deacon <will@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 01/12] ACPI/IORT: Make iort_match_node_callback walk the ACPI namespace for NC
Date: Wed, 1 Jul 2020 17:12:35 +0100
Message-ID: <952a6720-f401-1441-5548-5b40cfc76d3a@arm.com> (raw)
In-Reply-To: <4817d766-0437-5356-a0b9-97b111d4cae2@huawei.com>

On 2020-06-30 14:04, Hanjun Guo wrote:
> On 2020/6/30 18:24, Lorenzo Pieralisi wrote:
>> On Tue, Jun 30, 2020 at 11:06:41AM +0800, Hanjun Guo wrote:
>>
>> [...]
>>
>>>> For devices that aren't described in the DSDT - IORT translations
>>>> are determined by their ACPI parent device. Do you see/Have you
>>>> found any issue with this approach ?
>>>
>>> The spec says "Describes the IO relationships between devices
>>> represented in the ACPI namespace.", and in section 3.1.1.3 Named
>>> component node, it says:
>>
>> PCI devices aren't necessarily described in the ACPI namespace and we
>> still use IORT to describe them - through the RC node.
>>
>>> "Named component nodes are used to describe devices that are also
>>> included in the Differentiated System Description Table (DSDT). See
>>> [ACPI]."
>>>
>>> So from my understanding, the IORT spec for now, can only do ID
>>> translations for devices in the DSDT.
>>
>> I think you can read this multiple ways but this patch does not
>> change this concept. What changes, is applying parent's node IORT
>> mapping to child nodes with no associated DSDT nodes, it is the
>> same thing we do with PCI and the _DMA method - we could update
>> the wording in the specs if that clarifies but I don't think this
>> deliberately disregards the specifications.
> 
> I agree, but it's better to update the wording of the spec.
> 
>>
>>>>> For a platform device, if I use its parent's full path name for
>>>>> its named component entry, then it will match, but this will violate
>>>>> the IORT spec.
>>>>
>>>> Can you elaborate on this please I don't get the point you
>>>> are making.
>>>
>>> For example, device A is not described in DSDT so can't represent
>>> as a NC node in IORT. Device B can be described in DSDT and it
>>> is the parent of device A, so device B can be represented in IORT
>>> with memory access properties and node flags with Substream width
>>> and Stall supported info.
>>>
>>> When we trying to translate device A's ID, we reuse all the memory
>>> access properties and node flags from its parent (device B), but
>>> will it the same?
>>
>> I assume so why wouldn't it be ? Why would be describe them in
>> a parent-child relationship if that's not how the system looks like
>> in HW ?
> 
> The point I'm making is that I'm not sure all the memory access and
> stall properties are the same for the parent and the device itself.

Is that even a valid case though? The principal thing we want to 
accommodate here is when device B *is* the one accessing memory, either 
because it is a bridge with device A sat behind it, or because device A 
is actually just some logical function or subset of physical device B.

If the topology is such that device A is a completely independent device 
with its own path to memory such that it could have different 
properties, I would expect that it *should* be described in DSDT, and I 
can't easily think of a good reason why it wouldn't be. I'm also 
struggling to imagine how it might even have an ID that had to be 
interpreted in the context of device B if it wasn't one of the cases 
above :/

I don't doubt that people could - or maybe even have - come up with crap 
DSDT bindings that don't represent the hardware sufficiently accurately, 
but I'm not sure that should be IORT's problem...

Robin.

>> Do you have a specific example in mind that we should be aware of ?
>>
>>> So the IORT spec don't support this, at least it's pretty vague
>>> I think.
>>
>> I think that's a matter of wording, it can be updated if it needs be,
>> reach out if you see any issue with the current approach please.
> 
> If the all the properties for parent and device itself are the same,
> I have no strong opinion for this patch, but it's better to update
> the wording of the spec as well.
> 
> Thanks
> Hanjun
> 
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