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

[Marco Pagani <marco.pagani@linux.dev>]

On 23/09/2026 17:09, Krzysztof Kozlowski wrote:
> 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.

Sorry, I couldn't have imagined that missing ABI documentation in a
"proof of concept" RFC would be so detrimental while asking for a
high-level opinion.


> 
>>
>> - 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.

Glad to know the problem is already being solved.


Thanks,
Marco