linux-kernel - Re: [PATCH V7 0/3] Generate device tree node for pci devices

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Date:   Sun, 26 Feb 2023 16:38:58 -0600
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 V7 0/3] Generate device tree node for pci devices

Hi Clément, Hi Lizhi,

On 1/19/23 21:02, 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. Second, it allows reuse of a OF
> compatible driver -- often used in SoC platforms -- in a PCI host based
> system.
> 
> There are 2 series devices rely on this patch:
> 
>   1) Xilinx Alveo Accelerator cards (FPGA based device)
>   2) Microchip LAN9662 Ethernet Controller
> 

Digging back through some history:

>      Please see: https://lore.kernel.org/lkml/20220427094502.456111-1-clement.leger@bootlin.com/

(I am selectively pulling two fragments, see the above link for the
full email.)

Includes the following:

   A driver using this support was added and can be seen at [3]. This
   driver embeds a builtin overlay and applies it to the live tree using
   of_overlay_fdt_apply_to_node(). An interrupt driver is also included and

and

   This series was tested on a x86 kernel using CONFIG_OF under a virtual
   machine using PCI passthrough.

   Link: [1] https://lore.kernel.org/lkml/YhQHqDJvahgriDZK@lunn.ch/t/
   Link: [2] https://lore.kernel.org/lkml/20220408174841.34458529@fixe.home/T/
   Link: [3] https://github.com/clementleger/linux/tree/lan966x/of_support

Following link 3 to see how the driver implemented the concept, I arrived
at a git tree, with the commit be42efa "mfd: lan966x: add pci driver",
and have been looking at the code there.

Clément, is this still the best example of a driver implementation that
would use the framework proposed in the "[PATCH V7 0/3] Generate device
tree node for pci devices" patch series? And this is the driver for the
device listed as item 2 above "2) Microchip LAN9662 Ethernet Controller"?

-Frank

> 
> 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 three patches.

< snip >