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

[Frank Rowand <frowand.list@gmail.com>]

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.

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.  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.

> To use device tree overlay, there are three obvious
> problems that need to be resolved.
> 
> First, we need to create a base tree for non-DT system such as x86_64. A
> patch series has been submitted for this:
> https://lore.kernel.org/lkml/20220624034327.2542112-1-frowand.list@gmail.com/
> https://lore.kernel.org/lkml/20220216050056.311496-1-lizhi.hou@xilinx.com/
> 
> Second, a device tree node corresponding to the PCI endpoint is required
> for overlaying the flattened device tree blob for that PCI endpoint.
> Because PCI is a self-discoverable bus, a device tree node is usually not
> created for PCI devices. This series adds support to generate a device
> tree node for a PCI device which advertises itself using PCI quirks
> infrastructure.
> 
> Third, we need to generate device tree nodes for PCI bridges since a child
> PCI endpoint may choose to have a device tree node created.
> 
> This patch series is made up of two patches.
> 
> The first patch is adding OF interface to allocate an OF node. It is copied
> from:
> https://lore.kernel.org/lkml/20220620104123.341054-5-clement.leger@bootlin.com/
> 
> The second patch introduces a kernel option, CONFIG_PCI_OF. When the option
> is turned on, the kernel will generate device tree nodes for all PCI
> bridges unconditionally. The patch also shows how to use the PCI quirks
> infrastructure, DECLARE_PCI_FIXUP_FINAL to generate a device tree node for
> a device. Specifically, the patch generates a device tree node for Xilinx
> Alveo U50 PCIe accelerator device. The generated device tree nodes do not
> have any property. Future patches will add the necessary properties.
> 
> Clément Léger (1):
>   of: dynamic: add of_node_alloc()
> 
> Lizhi Hou (1):
>   pci: create device tree node for selected devices
> 
>  drivers/of/dynamic.c        |  50 +++++++++++++----
>  drivers/pci/Kconfig         |  11 ++++
>  drivers/pci/bus.c           |   2 +
>  drivers/pci/msi/irqdomain.c |   6 +-
>  drivers/pci/of.c            | 106 ++++++++++++++++++++++++++++++++++++
>  drivers/pci/pci-driver.c    |   3 +-
>  drivers/pci/pci.h           |  16 ++++++
>  drivers/pci/quirks.c        |  11 ++++
>  drivers/pci/remove.c        |   1 +
>  include/linux/of.h          |   7 +++
>  10 files changed, 200 insertions(+), 13 deletions(-)
>