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

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Date:   Wed, 14 Sep 2022 13:08:22 -0500
From:   Rob Herring <robh@...nel.org>
To:     Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
Cc:     Lizhi Hou <lizhi.hou@....com>, PCI <linux-pci@...r.kernel.org>,
        devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Clément Léger <clement.leger@...tlin.com>,
        "Zhen, Max" <max.zhen@....com>,
        "Santan, Sonal" <sonal.santan@....com>,
        "Liu, Larry" <larry.liu@....com>, "Xu, Brian" <brian.xu@....com>,
        Stefano Stabellini <stefano.stabellini@...inx.com>,
        Tom Rix <trix@...hat.com>
Subject: Re: [PATCH RFC 0/2] Generate device tree node for pci devices

On Wed, Sep 14, 2022 at 8:35 AM Jeremi Piotrowski
<jpiotrowski@...ux.microsoft.com> wrote:
>
> 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?

It would integrate easily unless the sub-devices you are targeting
have drivers already which are not PCI drivers. Sure, we could add PCI
support to them, but that could be a lot of churn.

There are also usecases where we don't get to change the h/w.

Rob