fpga: region: Add support for FPGA region variants

This RFC proposes a proof-of-concept implementation of FPGA region
variants, a mechanism that introduces a common way to handle
dynamic partial reconfiguration from userspace. The proposed approach
is safe and aligned with the mainline kernel's stance on hardware
management by constraining the hardware to a mutually exclusive set
of configurations (variants) defined upfront. This is a realistic
assumption for FPGAs, as regions are typically statically defined and
synthesized during the system design phase. To keep the architecture
realistic, the following additional constraints are introduced:
(i) variants cannot be nested, and (ii) variants cannot contain FPGA
bridges.

The interface and core logic for the variant mechanism are defined
in the fpga-region and implemented in a backwards-compatible way.
The fpga-region now optionally exports sysfs attributes that allow
the user to reconfigure a region that supports variants by selecting
one variant a list of pre-defined variants. Concrete regions can enable
variant support by implementing the new apply_variant and remove_variant
methods and adding them to fpga_region_info before registration.

As part of this RFC, the of-fpga-region concrete region has been extended
to implement the variant interface. Variants are statically specified in
the device tree using an fpga-variants node. Additionally, it introduces
a firmware-cached property to cache bitstreams in memory, enabling a fast
reconfiguration path for real-time (latency-sensitive) applications.

Below is an example of how variants can be statically defined in the
device tree under this architecture:

fake_mgr: fpga-mgr@0 {
        compatible = "linux,fake-fpga-mgr";
};

fpga_region: fpga-region@0 {
        compatible = "fpga-region";
        #address-cells = <2>;
        #size-cells = <2>;
        ranges;

        /* FPGA region properties */
        fpga-mgr = <&fake_mgr>;
        partial-fpga-config;
        region-unfreeze-timeout-us = <10000>;

        /* Base variant */
        base-variant = "variant-1";

        /* Variants container node */
        fpga-variants {
                #address-cells = <2>;
                #size-cells = <2>;
                ranges;

                variant-1 {
                        firmware-name = "variant1-image.bin";

                        #address-cells = <2>;
                        #size-cells = <2>;
                        ranges;

                        variant_1_ip: ip_1@10000 {
                                compatible = "fake,ip_1";
                                reg = <0x0 0x10000 0x0 0x1000>;
                        };
                };

                variant-2 {
                        firmware-name = "variant2-image.bin";
                        firmware-cached;

                        #address-cells = <2>;
                        #size-cells = <2>;
                        ranges;

                        variant_2_ip: ip_2@10000 {
                                compatible = "fake,ip_2";
                                reg = <0x0 0x20000 0x0 0x1000>;
                        };
                };
        };
};

Marco Pagani (2):
  fpga: region: Add support for FPGA region variants
  fpga: of-fpga-region: Add support for region variants

 drivers/fpga/fpga-region.c       | 171 +++++++++++++++++++
 drivers/fpga/of-fpga-region.c    | 272 ++++++++++++++++++++++++++++++-
 include/linux/fpga/fpga-region.h |  32 ++++
 3 files changed, 473 insertions(+), 2 deletions(-)

-- 
2.54.0