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

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Message-ID: <1d9faa2e-e3fc-d104-c85f-4035233848d6@gmail.com>
Date:   Tue, 13 Sep 2022 02:00:47 -0500
From:   Frank Rowand <frowand.list@...il.com>
To:     Lizhi Hou <lizhi.hou@....com>, linux-pci@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        robh@...nel.org, helgaas@...nel.org
Cc:     clement.leger@...tlin.com, max.zhen@....com, sonal.santan@....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 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. 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(-)
> 

The patch description leaves out the most important piece of information.

The device located at the PCI endpoint is implemented via FPGA
   - which is programmed after Linux boots (or somewhere late in the boot process)
      - (A) and thus can not be described by static data available pre-boot because
            it is dynamic (and the FPGA program will often change while the Linux
            kernel is already booted
      - (B) can be described by static data available pre-boot because the FPGA
            program will always be the same for this device on this system

I am not positive what part of what I wrote above is correct and would appreciate
some confirmation of what is correct or incorrect.

-Frank