[PATCH v12 7/7] coco: guest: arm64: Replace dummy CCA device with sysfs ABI

["Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org>]

The SMCCC firmware driver now creates the arm_smccc bus and instantiates
the CCA RSI device once the RSI ABI is discovered. The arm64-specific
arm-cca-dev platform device stub is therefore no longer needed.

However, userspace has used the arm-cca-dev platform device to detect Arm
CCA Realm guests [1]. Removing it without a replacement would break that
detection and would also leave userspace depending on kernel device-model
details.

Add /sys/firmware/cca/realm_guest as a stable ABI for detecting whether
the kernel is running as an Arm CCA Realm guest. Create the file only in
Realm world, where it reads 1.

Remove the dummy arm-cca-dev registration now that userspace has a
dedicated CCA Realm guest indicator, and document the new ABI in
Documentation/ABI/testing/sysfs-firmware-cca.

Link: https://lore.kernel.org/all/4a7d84b2-2ec4-4773-a2d5-7b63d5c683cf@arm.com
Reviewed-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
---
 Documentation/ABI/testing/sysfs-firmware-cca |  7 ++++
 drivers/firmware/arm_rmm/rsi.c               | 43 +++++++++++++++-----
 2 files changed, 39 insertions(+), 11 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-firmware-cca

diff --git a/Documentation/ABI/testing/sysfs-firmware-cca b/Documentation/ABI/testing/sysfs-firmware-cca
new file mode 100644
index 000000000000..9cc355724f94
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-firmware-cca
@@ -0,0 +1,7 @@
+What:		/sys/firmware/cca/realm_guest
+Date:		May 2026
+Contact:	Linux ARM Kernel Mailing list <linux-arm-kernel@lists.infradead.org>
+Description:	Read-only. Present only when the kernel is running as an
+		Arm Confidential Compute Architecture (CCA) Realm guest, in
+		which case it reads 1. If the file does not exist, the kernel
+		is not running as a Realm guest.
diff --git a/drivers/firmware/arm_rmm/rsi.c b/drivers/firmware/arm_rmm/rsi.c
index 2dd69febedbd..425b27c0cad4 100644
--- a/drivers/firmware/arm_rmm/rsi.c
+++ b/drivers/firmware/arm_rmm/rsi.c
@@ -7,14 +7,16 @@
 #include <linux/memblock.h>
 #include <linux/psci.h>
 #include <linux/swiotlb.h>
-#include <linux/platform_device.h>
 #include <linux/arm-rsi-cmds.h>
+#include <linux/kobject.h>
+#include <linux/sysfs.h>
 
 #include <asm/io.h>
 #include <asm/mem_encrypt.h>
 #include <asm/pgtable.h>
 
 static struct realm_config config;
+static struct kobject *cca_kobj;
 
 unsigned long prot_ns_shared;
 EXPORT_SYMBOL(prot_ns_shared);
@@ -182,17 +184,36 @@ void __init arm64_rsi_init(void)
 	static_branch_enable(&rsi_present);
 }
 
-static struct platform_device rsi_dev = {
-	.name = "arm-cca-dev",
-	.id = PLATFORM_DEVID_NONE
+static ssize_t realm_guest_show(struct kobject *kobj,
+		struct kobj_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "1\n");
+}
+
+static struct kobj_attribute cca_realm_guest =
+	__ATTR(realm_guest, 0444, realm_guest_show, NULL);
+
+static const struct attribute *cca_realm_attrs[] = {
+	&cca_realm_guest.attr,
+	NULL
 };
 
-static int __init arm64_create_dummy_rsi_dev(void)
+static int __init realm_sysfs_init(void)
 {
-	if (is_realm_world() &&
-	    platform_device_register(&rsi_dev))
-		pr_err("failed to register rsi platform device\n");
-	return 0;
-}
+	int ret;
 
-arch_initcall(arm64_create_dummy_rsi_dev)
+	if (!is_realm_world())
+		return 0;
+
+	cca_kobj = kobject_create_and_add("cca", firmware_kobj);
+	if (!cca_kobj)
+		return -ENOMEM;
+
+	ret = sysfs_create_files(cca_kobj, cca_realm_attrs);
+	if (!ret)
+		return 0;
+
+	kobject_put(cca_kobj);
+	return ret;
+}
+device_initcall(realm_sysfs_init);
-- 
2.43.0