Date:   Tue, 13 Sep 2022 14:02:49 -0700
From:   Lizhi Hou <lizhi.hou@....com>
To:     Frank Rowand <frowand.list@...il.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 9/13/22 10:41, Frank Rowand wrote:
> On 9/13/22 12:10, Lizhi Hou wrote:
>> On 9/13/22 00:00, Frank Rowand wrote:
>>> 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.
>> There are 2 series devices rely on this patch:
>>
>>      1) Xilinx Alveo Accelerator cards (FPGA based device)
>>
>>      2) lan9662 PCIe card
>>
>>            please see: https://lore.kernel.org/lkml/20220427094502.456111-1-clement.leger@bootlin.com/
> Thanks.  Please include this information in future versions of the patch series.
>
> For device 2 I have strongly recommended using pre-boot apply of the overlay to the base
> device tree.  I realize that this suggestion is only a partial solution if one wants to
> use hotplug to change system configuration (as opposed to using hotplug only to replace
> an existing device (eg a broken device) with another instance of the same device).  I
> also realize that this increased the system administration overhead.  On the other hand
> an overlay based solution is likely to be fragile and possibly flaky.
Can you clarify the pre-boot apply approach? How will it work for PCI 
devices?
>
>> For Xilinx Alveo device, it is (A). The FPGA partitions can be programmed dynamically after boot.
> I looked at the Xilinx Alveo web page, and there are a variety of types of Alveo cards
> available.  So the answer to my next question may vary by type of card.
>
> Is it expected that the fpga program on a given card will change frequently (eg multiple
> times per day), where the changed program results in a new device that would require a
> different hardware description in the device tree?

Different images may be loaded to a FPGA partition several times a day. 
The PCI topology (Device IDs, BARs, MSIx, etc) does not change. New IPs 
may appear (and old IPs may disappear) on the BARs when a new image is 
loaded. We would like to use flattened device tree to describe the IPs 
on the BARs.


Thanks,

Lizhi

>
> Or is the fpga program expected to change on an infrequent basis (eg monthly, quarterly,
> annually), in the same way as device firmware and operating systems are updated on a regular
> basis for bug fixes and new functionality?
>
>
>>
>> Thanks,
>>
>> Lzhi
>>
>>> -Frank

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