dibs: Define dibs loopback

The first stage of loopback-ism was implemented as part of the
SMC module [1]. Now that we have the dibs layer, provide access to a
dibs_loopback device to all dibs clients.

This is the first step of moving loopback-ism from net/smc/smc_loopback.*
to drivers/dibs/dibs_loopback.*. One global structure lo_dev is allocated
and added to the dibs devices. Follow-on patches will move functionality.

Same as smc_loopback, dibs_loopback is provided by a config option.
Note that there is no way to dynamically add or remove the loopback
device. That could be a future improvement.

When moving code to drivers/dibs, replace ism_ prefix with dibs_ prefix.
As this is mostly a move of existing code, copyright and authors are
unchanged.

Link: https://lore.kernel.org/lkml/20240428060738.60843-1-guwen@linux.alibaba.com/ [1]
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://patch.msgid.link/20250918110500.1731261-7-wintera@linux.ibm.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

arch/s390/configs/debug_defconfig

@@ -121,6 +121,7 @@ CONFIG_UNIX_DIAG=m
 CONFIG_XFRM_USER=m
 CONFIG_NET_KEY=m
 CONFIG_DIBS=y
+CONFIG_DIBS_LO=y
 CONFIG_SMC_DIAG=m
 CONFIG_SMC_LO=y
 CONFIG_INET=y

arch/s390/configs/defconfig

@@ -112,6 +112,7 @@ CONFIG_UNIX_DIAG=m
 CONFIG_XFRM_USER=m
 CONFIG_NET_KEY=m
 CONFIG_DIBS=y
+CONFIG_DIBS_LO=y
 CONFIG_SMC_DIAG=m
 CONFIG_SMC_LO=y
 CONFIG_INET=y

drivers/dibs/Kconfig

@@ -10,3 +10,14 @@ config DIBS
 	  Select this option to provide the abstraction layer between
 	  dibs devices and dibs clients like the SMC protocol.
 	  The module name is dibs.
+
+config DIBS_LO
+	bool "intra-OS shortcut with dibs loopback"
+	depends on DIBS
+	default n
+	help
+	  DIBS_LO enables the creation of an software-emulated dibs device
+	  named lo which can be used for transferring data when communication
+	  occurs within the same OS. This helps in convenient testing of
+	  dibs clients, since dibs loopback is independent of architecture or
+	  hardware.

drivers/dibs/Makefile

@@ -5,3 +5,4 @@
 
 dibs-y += dibs_main.o
 obj-$(CONFIG_DIBS) += dibs.o
+dibs-$(CONFIG_DIBS_LO) += dibs_loopback.o

drivers/dibs/dibs_loopback.c

@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  Functions for dibs loopback/loopback-ism device.
+ *
+ *  Copyright (c) 2024, Alibaba Inc.
+ *
+ *  Author: Wen Gu <guwen@linux.alibaba.com>
+ *          Tony Lu <tonylu@linux.alibaba.com>
+ *
+ */
+
+#include <linux/dibs.h>
+#include <linux/slab.h>
+#include <linux/types.h>
+
+#include "dibs_loopback.h"
+
+/* global loopback device */
+static struct dibs_lo_dev *lo_dev;
+
+static void dibs_lo_dev_exit(struct dibs_lo_dev *ldev)
+{
+	dibs_dev_del(ldev->dibs);
+}
+
+static int dibs_lo_dev_probe(void)
+{
+	struct dibs_lo_dev *ldev;
+	struct dibs_dev *dibs;
+	int ret;
+
+	ldev = kzalloc(sizeof(*ldev), GFP_KERNEL);
+	if (!ldev)
+		return -ENOMEM;
+
+	dibs = dibs_dev_alloc();
+	if (!dibs) {
+		kfree(ldev);
+		return -ENOMEM;
+	}
+
+	ldev->dibs = dibs;
+
+	ret = dibs_dev_add(dibs);
+	if (ret)
+		goto err_reg;
+	lo_dev = ldev;
+	return 0;
+
+err_reg:
+	/* pairs with dibs_dev_alloc() */
+	kfree(dibs);
+	kfree(ldev);
+
+	return ret;
+}
+
+static void dibs_lo_dev_remove(void)
+{
+	if (!lo_dev)
+		return;
+
+	dibs_lo_dev_exit(lo_dev);
+	/* pairs with dibs_dev_alloc() */
+	kfree(lo_dev->dibs);
+	kfree(lo_dev);
+	lo_dev = NULL;
+}
+
+int dibs_loopback_init(void)
+{
+	return dibs_lo_dev_probe();
+}
+
+void dibs_loopback_exit(void)
+{
+	dibs_lo_dev_remove();
+}

drivers/dibs/dibs_loopback.h

@@ -0,0 +1,38 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ *  dibs loopback (aka loopback-ism) device structure definitions.
+ *
+ *  Copyright (c) 2024, Alibaba Inc.
+ *
+ *  Author: Wen Gu <guwen@linux.alibaba.com>
+ *          Tony Lu <tonylu@linux.alibaba.com>
+ *
+ */
+
+#ifndef _DIBS_LOOPBACK_H
+#define _DIBS_LOOPBACK_H
+
+#include <linux/dibs.h>
+#include <linux/types.h>
+#include <linux/wait.h>
+
+#if IS_ENABLED(CONFIG_DIBS_LO)
+
+struct dibs_lo_dev {
+	struct dibs_dev *dibs;
+};
+
+int dibs_loopback_init(void);
+void dibs_loopback_exit(void);
+#else
+static inline int dibs_loopback_init(void)
+{
+	return 0;
+}
+
+static inline void dibs_loopback_exit(void)
+{
+}
+#endif
+
+#endif /* _DIBS_LOOPBACK_H */

drivers/dibs/dibs_main.c

@@ -15,6 +15,8 @@
 #include <linux/err.h>
 #include <linux/dibs.h>
 
+#include "dibs_loopback.h"
+
 MODULE_DESCRIPTION("Direct Internal Buffer Sharing class");
 MODULE_LICENSE("GPL");
 
@@ -96,14 +98,21 @@ EXPORT_SYMBOL_GPL(dibs_dev_del);
 
 static int __init dibs_init(void)
 {
+	int rc;
+
 	memset(clients, 0, sizeof(clients));
 	max_client = 0;
 
-	return 0;
+	rc = dibs_loopback_init();
+	if (rc)
+		pr_err("%s fails with %d\n", __func__, rc);
+
+	return rc;
 }
 
 static void __exit dibs_exit(void)
 {
+	dibs_loopback_exit();
 }
 
 module_init(dibs_init);