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

[Frank Rowand]

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@xxxxxxxxxx/
>>>>> https://lore.kernel.org/lkml/20220427094502.456111-1-clement.leger@xxxxxxxxxxx/
>>>>>
>>>>> 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 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@xxxxxxxxxxx/
>>>>>
>>>>> 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@xxxxxxxxxxx/
>> 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").

-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