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

[Sonal Santan]

On 9/26/22 15:44, Rob Herring wrote:
> On Fri, Sep 16, 2022 at 6:15 PM Frank Rowand <frowand.list@xxxxxxxxx> 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@xxxxxxxxxx/
> > > https://lore.kernel.org/lkml/20220427094502.456111-1-clement.leger@xxxxxxxxxxx/
> > >
> > > 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