Re: [PATCH 0/3] add dynamic PCI device of_node creation for overlay

[Frank Rowand]

On 5/9/22 12:00, Andy Shevchenko wrote:
> On Mon, May 09, 2022 at 06:09:17PM +0200, Clément Léger wrote:
>> Le Mon, 9 May 2022 10:56:36 -0500,
>> Frank Rowand <frowand.list@xxxxxxxxx> a écrit :
> 
> ...
> 
>>> On the surface, it appears that your need might be well met by having
>>> a base devicetree that describes all of the pcie nodes, but with each
>>> node having a status of "disabled" so that they will not be used.
>>> Have a devicetree overlay describing the pcie card (as you proposed),
>>> where the overlay also includes a status of "ok" for the pcie node.
>>> Applying the overlay, with a method of redirecting the target to a
>>> specific pcie node would change the status of the pcie node to enable
>>> its use.  (You have already proposed a patch to modify
>>> of_overlay_fdt_apply() to allow a modified target, so not a new
>>> concept from me.)  My suggestion is to apply the overlay devicetree
>>> to the base devicetree before the combined FDT devicetree is passed
>>> to the kernel at boot.  The overlay apply could be done by several
>>> different entities.  It could be before the bootloader executes, it
>>> could be done by the bootloader, it could be done by a shim between
>>> the bootloader and the kernel.  This method avoids all of the issues
>>> of applying an overlay to a running system that I find problematic.
>>> It is also a method used by the U-boot bootloader, as an example.
>>
>> Ok, that is actually possible on a system that is given a device-tree
>> by the bootloader. But on a system that is desrcibed using ACPI (such
>> as the x86), this is much more difficult (at least to my knowledge)...
>> We want this feature to be easy to use for the end user. Adding such
>> configuration which also differs between various architecture is
>> clearly not so easy to setup.
>>
>> Moreover, since the PCI is meant to be "Plug and Play", such
>> configuration would completely break that. If the user switches the
>> PCIe card from one slot to another, the bootloader configuration will
>> need to be modified. This seems a big no way for me (and for the user).
> 
> The main problem here is that Linux does not support hotplugging for the
> devices behind non-hotpluggable buses. You need to develop something to
> say that the device tree (in terms of hardware) can morph at run-time
> transparently to the user. I think the closest one is what FPGA does,
> or at least should do.

That is something I was not aware of yet.  Is the card in question a
hotpluggable card?  Do the systems that you anticipate plugging the
card into support hotplug?

-Frank

> 
>>> The other big issue is mixing ACPI and devicetree on a single system.
>>> Historically, the Linux devicetree community has not been receptive
>>> to the ides of that mixture.  Your example might be a specific case
>>> where the two can be isolated from each other, or maybe not.  (For
>>> disclosure, I am essentially ACPI ignorant.)  I suspect that mixing
>>> ACPI and devicetree is a recipe for disaster in the general case.
>>
>> Agreed, on that fact, it did raised some eyebrows, and it was for that
>> specific concern that initially, I proposed the fwnode solution.
>> Honestly, the fwnode conversion represent a lot of work (hundreds of
>> lines easily) + requires a conversion of all the subsystem that are not
>> fwnode ready (spoiler: almost all of them are not ready). 
> 
> In either case you need to provide a format that would be suitable for
> DT-based as well as ACPI-based platforms.
>