[PATCH 1/2] Documentation: ABI: add sysfs interface for ZynqMP CSU registers

[Ronak Jain]

Document the new sysfs interface that exposes Configuration Security
Unit (CSU) registers through the zynqmp-firmware driver.

The interface is available under:

  /sys/devices/platform/firmware:zynqmp-firmware/csu_registers/

The CSU registers are discovered at boot time using the PM_QUERY_DATA
firmware API. The following registers are currently supported:

  - multiboot     (CSU_MULTI_BOOT)
  - idcode        (CSU_IDCODE, read-only)
  - pcap-status   (CSU_PCAP_STATUS, read-only)

Read operations use the existing IOCTL_READ_REG firmware interface,
while write operations use IOCTL_MASK_WRITE_REG.

Access control is enforced by the firmware. Write attempts to
read-only registers are rejected by firmware even though the sysfs file
permissions allow writes.

Document the ABI entry accordingly.

Signed-off-by: Ronak Jain <ronak.jain@xxxxxxx>
---
 .../ABI/stable/sysfs-driver-firmware-zynqmp   | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/Documentation/ABI/stable/sysfs-driver-firmware-zynqmp b/Documentation/ABI/stable/sysfs-driver-firmware-zynqmp
index c3fec3c835af..f537f7d9bb55 100644
--- a/Documentation/ABI/stable/sysfs-driver-firmware-zynqmp
+++ b/Documentation/ABI/stable/sysfs-driver-firmware-zynqmp
@@ -254,3 +254,36 @@ Description:
 		The expected result is 500.
 
 Users:		Xilinx
+
+What:		/sys/devices/platform/firmware\:zynqmp-firmware/csu_registers/*
+Date:		March 2026
+KernelVersion:	7.1
+Contact:	"Ronak Jain" <ronak.jain@xxxxxxx>
+Description:
+		Read/Write CSU (Configuration Security Unit) registers.
+
+		This interface provides dynamic access to CSU registers that are
+		discovered from the firmware at boot time using PM_QUERY_DATA API.
+
+		The supported registers are:
+
+		- multiboot: CSU_MULTI_BOOT register
+		- idcode: CSU_IDCODE register (read-only)
+		- pcap-status: CSU_PCAP_STATUS register (read-only)
+
+		Read operations use the existing IOCTL_READ_REG API.
+		Write operations use the existing IOCTL_MASK_WRITE_REG API.
+
+		The firmware enforces access control - read-only registers will reject
+		write attempts even though the sysfs permissions show write access.
+
+		Usage for reading::
+
+		    # cat /sys/devices/platform/firmware\:zynqmp-firmware/csu_registers/multiboot
+		    # cat /sys/devices/platform/firmware\:zynqmp-firmware/csu_registers/idcode
+
+		Usage for writing (mask and value are in hexadecimal)::
+
+		    # echo 0xFFFFFFF 0x0 > /sys/devices/platform/firmware\:zynqmp-firmware/csu_registers/multiboot
+
+Users:		Xilinx/AMD
-- 
2.34.1