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

[Clément Léger]

Le Sun, 26 Feb 2023 16:38:58 -0600,
Frank Rowand <frowand.list@xxxxxxxxx> a écrit :

> 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@xxxxxxxxxxx/  
> 
> (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@xxxxxxx/t/
>    Link: [2] https://lore.kernel.org/lkml/20220408174841.34458529@xxxxxxxxx/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"?

Hi Frank,

The driver has slightly evolved to be based on Lizhi Patches and the
interrupt driver was reworked to be a standard platform driver. I'll
clean that up and push a new branch based on this work.

This driver is indeed the driver for the LAN9662 Ethernet Controller
which allows using the 2 SFPs ports and 2 RJ45 ports successfully (which
involves multiple subsystem and drivers).

While doing this work, I found multiple of_noderefcount issues which I
fixed and that are currently being reviewed. I won't be surprised if
there are other lying around in various part of the kernel. Just saying
so you know there is actually effort to make that more robust.

Clément

> 
> -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@xxxxxxxxx/
> > https://lore.kernel.org/lkml/20220216050056.311496-1-lizhi.hou@xxxxxxxxxx/
> > 
> > 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 >
> 



-- 
Clément Léger,
Embedded Linux and Kernel engineer at Bootlin
https://bootlin.com