[RFC 0/1]Add user space interaction for FPGA programming

[Nava kishore Manne]

The existing FPGA manager subsystem didn't have any user space interface
(other than the status/state in sysfs) in Kernel. Basically, FPGAs are
semiconductor devices that can be reprogrammed for desired hardware
functionality.

FPGAs can be reprogrammed at runtime with different types of logic and IPs
as per user need and hence there is a need to use device tree overlays for
removing/updating/adding the devices at runtime for the IPs/controllers
that are present in FPGA. But we don't have any user interface in kernel
for updating the device tree at runtime.

Sometime back there was a series sent by Pantelis Antoniou
(https://lore.kernel.org/lkml/1414528565-10907-4-git-send-email-pantelis.antoniou@xxxxxxxxxxxx/).

This patch introduced a user interface configfs for Device Tree overlays,
a method of dynamically altering the kernel's live Device Tree. However,
this patch series was not accepted in mainline due to various concerns.
For more details refer to this link:
https://elinux.org/Frank%27s_Evolving_Overlay_Thoughts#issues_and_what_needs_to_be_completed_--_Not_an_exhaustive_list

One of the major valid concerns that were raised with this configfs
interface was security. It provides a generic interface (Not specific
to the use cases) for modifying the live device tree.

In order to configure/program the FPGA devices, All the major vendors of
FPGA are using this configfs series as out-of-tree patch for configuring
the FPGAs and there was never an attempt to introduce a generic interface
to configure/program the FPGA in upstream and hence upstream kernel ended
up in not having proper support for FPGAs configure/program.
This series tries to address this gap of FPGA programmability by providing
a new sysfs interface to the user.

This newly added sysfs interface uses Device Tree Overlay (DTO) files to
configure/reprogram an FPGA while an operating system is running and it
restrict the overlay's subsystem usage only to FPGA regions in order to
mitigate the major security concern with configfs.This proposed solution
will not change the existing sequence when a DT overlay that
targets an FPGA Region is applied.
	- Disable appropriate FPGA bridges.
	- Program the FPGA using the FPGA manager.
	- Enable the FPGA bridges.
	- The Device Tree overlay is accepted into the live tree.
	- Child devices are populated.

When the overlay is removed, the child nodes will be removed, and the FPGA
Region will disable the bridges.

Usage:
To configure/reprogram an FPGA region:
echo "fpga.dtbo" > /sys/class/fpga_region/<region>/device/load

To remove an FPGA region:
echo "fpga.dtbo" > /sys/class/fpga_region/<region>/device/remove

To get an FPGA region status:
cat /sys/class/fpga_region/<region>/device/status

Nava kishore Manne (1):
  of-fpga-region: Add sysfs interface support for FPGA configuration

 .../ABI/testing/sysfs-class-of-fpga-region    | 30 ++++++
 MAINTAINERS                                   |  1 +
 drivers/fpga/fpga-region.c                    |  4 +-
 drivers/fpga/of-fpga-region.c                 | 92 +++++++++++++++++++
 include/linux/fpga/fpga-region.h              | 15 +++
 5 files changed, 141 insertions(+), 1 deletion(-)
 create mode 100644 Documentation/ABI/testing/sysfs-class-of-fpga-region

-- 
2.34.1