Re: [RFC PATCH] fpga: region: Add support for FPGA region variants

[Krzysztof Kozlowski <krzk@kernel.org>]

On 23/09/2026 16:30, Marco Pagani wrote:
> 
> 
> On 18/09/2026 19:36, Xu Yilun wrote:
>> On Fri, Sep 18, 2026 at 05:07:07PM +0200, Marco Pagani wrote:
>>>
>>> Hi Yilun,
>>>
>>> Is it okay with you if I CC the DT people to ask for an opinion?
>>
>> Of course. It's good to know in which case could a DTO be applied.
>>
>> But to be clear, I think that only affects how we accept and apply the
>> image-DTO pair. For now, I basically don't want a varient selection
>> interface.
> 
> 
> Hello Rob, Saravana, Krzysztof, Conor,
> 
> I'm CC-ing you on this thread as OF/DT maintainers to kindly ask
> for your input on this debate we are having about how to implement
> userspace FPGA reconfiguration, an important feature currently
> lacking in mainline kernel.
> 
> https://lore.kernel.org/all/20260608164247.1998417-1-marco.pagani@linux.dev
> 
> To recap: modern FPGAs allow portions of the fabric to be reconfigured
> at runtime to dynamically swap soft IPs. Usually, these IPs are
> connected through a non-discoverable on-chip bus like AMBA AXI.
> From a kernel perspective, this means the FPGA device configuration
> image (bitstream) must be paired with a DT fragment that describes
> the new topology.
> 
> In this thread, we are essentially debating between two approaches for
> implementing FPGA userspace reconfiguration at runtime:
> 
> - Statically defined Variants (this RFC): Userspace can change FPGA
>   configuration by selecting from a pre-validated set of variants
>   (DT fragments) baked into the base DT.

I don't know what DT fragments are, maybe you meant overlays? But they
are not baked into the base DT.

There are no bindings here and no DT maintainers were CCed on this RFC,
so I really do not get how this patch can enable something like that.

Linked URL shows some undocumented ABI, so obviously this would be a no
go. And RFC prefix does not justify undocumented ABI. At least one
without clear explanation in the cover letter, why is this RFC and why
it cannot be merged/reviewed.


> 
> - Runtime DTOs (Nava's RFC): Userspace can change FPGA configuration
>   by loading DTOs that are validated at runtime.

If runtime overlays were working, this looks suitable to the problem of
runtime change of the hardware. This also fits hot-pluggable hardware
problem, which Bootlin is working on.


Best regards,
Krzysztof