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Subject: Re: [PATCH RFC 0/2] Generate device tree node for pci devices
To:     Frank Rowand <frowand.list@gmail.com>,
        Lizhi Hou <lizhi.hou@amd.com>, linux-pci@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        robh@kernel.org, helgaas@kernel.org
Cc:     clement.leger@bootlin.com, max.zhen@amd.com, sonal.santan@amd.com,
        larry.liu@amd.com, brian.xu@amd.com, stefano.stabellini@xilinx.com
References: <1661809417-11370-1-git-send-email-lizhi.hou@amd.com>
 <1d9faa2e-e3fc-d104-c85f-4035233848d6@gmail.com>
 <ca35a14d-501d-265e-b196-a87e1e994cd0@amd.com>
 <78211af5-171c-ef4f-a8c2-17f63dc479bc@gmail.com>
 <8bee1ddd-0a84-62cd-9beb-3bbb09d83de1@amd.com>
 <f042a8d7-fa2b-3e75-a2f2-6e6ba28a9b38@gmail.com>
From:   Tom Rix <trix@redhat.com>
Message-ID: <7393f2f3-3af5-edf9-4afb-8acc0a8db1e7@redhat.com>
Date:   Sat, 17 Sep 2022 11:36:24 -0700
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Frank,

On 9/16/22 7:23 PM, Frank Rowand wrote:
> On 9/13/22 16:02, Lizhi Hou wrote:
>> 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.
> That was kind of a non-answer.  I know that images _may_ change at
> some frequency.  I was trying to get a sense of whether the images
> were _likely_ to be changing on a frequent basis for these types
> of boards, or whether frequent image changes are likely to be a
> rare edge use case.
>
> If there is a good design for the 99.999% use case that does not
> support the 0.001% use case then it may be better to not create
> an inferior design that also supports the 0.001% use case.
>
> I hope that gives a better idea of the reason why I was asking the
> question and how the answer could impact design and implementation
> decisions.
>
> As a point of reference, some other fpga users have indicated a
> desire to change images many times per second.  The current driver
> and overlay architecture did not seem to me to be a good match to
> that use case (depending on the definition of "many").

I would rather we cover 99.999% now.

My understanding is that the subdevices are flexible but fairly static 
and the frequency Lizhi mentions would cover development uses.

In production I would expect the image to change about once a year with 
the same order of magnitude as firmware.

Can you point me to a reference of a user case with high frequency 
images changing that also depends on pci io device changing?

Tom

> -Frank
>
>> 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