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

Jeremi Piotrowski
2022-09-14T13:35:20Z

On Mon, Aug 29, 2022 at 02:43:35PM -0700, 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. To use device tree overlay, there are three obvious
> problems that need to be resolved.

Hi Lizhi,

We all *love* "have you thought about xxx" questions but I would really like to
get your thoughts on this. An approach to this problem that I have seen in
various devices is to emulate a virtual pcie switch, and expose the "sub
devices" behind that. That way you can carve up the BAR space, each device has
its own config space and mapping of MSI-X vector to device becomes clear. This
approach also integrates well with other kernel infrastructure (IOMMU, hotplug).

This is certainly possible on reprogrammable devices but requires some more
FPGA resources - though I don't believe the added utilization would be
significant. What do you think of this kind of solution?

Jeremi

>
> 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(-)
>
> --
> 2.27.0
>