Date:   Fri, 30 Sep 2022 12:29:40 -0700
From:   Sonal Santan <sonal.santan@....com>
To:     Rob Herring <robh@...nel.org>,
        Frank Rowand <frowand.list@...il.com>
Cc:     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 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?

-Sonal

> Rob

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