linux-kernel - Re: [PATCH RFC 0/2] Generate device tree node for pci devices

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Date:   Fri, 7 Oct 2022 15:45:17 -0700
From:   Sonal Santan <sonal.santan@....com>
To:     Rob Herring <robh@...nel.org>
Cc:     Frank Rowand <frowand.list@...il.com>,
        Lizhi Hou <lizhi.hou@....com>, linux-pci@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        helgaas@...nel.org, clement.leger@...tlin.com, max.zhen@....com,
        larry.liu@....com, brian.xu@....com, stefano.stabellini@...inx.com,
        trix@...hat.com
Subject: Re: [PATCH RFC 0/2] Generate device tree node for pci devices

On 10/6/22 08:10, Rob Herring wrote:
> On Fri, Sep 30, 2022 at 2:29 PM Sonal Santan <sonal.santan@....com> wrote:
>>
>> On 9/26/22 15:44, Rob Herring wrote:
>>> On Fri, Sep 16, 2022 at 6:15 PM Frank Rowand <frowand.list@...il.com> wrote:
>>>>
>>>> On 8/29/22 16:43, Lizhi Hou wrote:
>>>>> This patch series introduces OF overlay support for PCI devices which
>>>>> primarily addresses two use cases. First, it provides a data driven method
>>>>> to describe hardware peripherals that are present in a PCI endpoint and
>>>>> hence can be accessed by the PCI host. An example device is Xilinx/AMD
>>>>> Alveo PCIe accelerators. Second, it allows reuse of a OF compatible
>>>>> driver -- often used in SoC platforms -- in a PCI host based system. An
>>>>> example device is Microchip LAN9662 Ethernet Controller.
>>>>>
>>>>> This patch series consolidates previous efforts to define such an
>>>>> infrastructure:
>>>>> https://lore.kernel.org/lkml/20220305052304.726050-1-lizhi.hou@xilinx.com/
>>>>> https://lore.kernel.org/lkml/20220427094502.456111-1-clement.leger@bootlin.com/
>>>>>
>>>>> Normally, the PCI core discovers PCI devices and their BARs using the
>>>>> PCI enumeration process. However, the process does not provide a way to
>>>>> discover the hardware peripherals that are present in a PCI device, and
>>>>> which can be accessed through the PCI BARs. Also, the enumeration process
>>>>> does not provide a way to associate MSI-X vectors of a PCI device with the
>>>>> hardware peripherals that are present in the device. PCI device drivers
>>>>> often use header files to describe the hardware peripherals and their
>>>>> resources as there is no standard data driven way to do so. This patch> series proposes to use flattened device tree blob to describe the
>>>>> peripherals in a data driven way.
>>>>
>>>>> Based on previous discussion, using
>>>>> device tree overlay is the best way to unflatten the blob and populate
>>>>> platform devices.
>>>>
>>>> I still do not agree with this statement.  The device tree overlay
>>>> implementation is very incomplete and should not be used until it
>>>> becomes more complete.  No need to debate this right now, but I don't want
>>>> to let this go unchallenged.
>>>
>>> Then we should remove overlay support. The only way it becomes more
>>> complete is having actual users.
>>>
>>> But really, whether this is the right solution to the problem is
>>> independent of the state of kernel overlay support.
>>>
>>>> If there is no base system device tree on an ACPI based system, then I
>>>> am not convinced that a mixed ACPI / device tree implementation is
>>>> good architecture.
>>>
>>> Most/all of this series is needed for a DT system in which the PCI
>>> devices are not populated in the DT.
>>>
>>>>    I might be more supportive of using a device tree
>>>> description of a PCI device in a detached device tree (not linked to
>>>> the system device tree, but instead freestanding).  Unfortunately the
>>>> device tree functions assume a single system devicetree, with no concept
>>>> of a freestanding tree (eg, if a NULL device tree node is provided to
>>>> a function or macro, it often defaults to the root of the system device
>>>> tree).  I need to go look at whether the flag OF_DETACHED handles this,
>>>> or if it could be leveraged to do so.
>>>
>>> Instead of worrying about a theoretical problem, we should see if
>>> there is an actual problem for a user.
>>>
>>> I'm not so worried about DT functions themselves, but places which
>>> have 'if ACPI ... else (DT) ...' paths.
>>>
>>
>> Bringing this thread back into focus. Any thoughts on how to move forward?
> 
> Reviewers raise concerns/issues and the submitters work to address
> them or explain why they aren't an issue. The submitter has to push
> things forward. That's how the process works.
> 
We are working on updating the patch set to address the feedback. The 
design is still based on device tree overlay infrastructure.

> As I noted, much of this is needed on a DT system with PCI device not
> described in DT. So you could split out any ACPI system support to
> avoid that concern for example. Enabling others to exercise these
> patches may help too. Perhaps use QEMU to create some imaginary
> device.
To verify this patch set, in addition to a x86_64/ACPI based system, we 
also have an AARCH64/DT QEMU setup where we have attached a physical 
Alveo device. We haven't run into any ACPI or DTO issues so far.

Perhaps we can introduce this feature in a phased manner where we first 
enable DT based platforms and then enable ACPI based platforms?

-Sonal
> 
> Rob