[RFC PATCH v3 4/6] char: mem: expose devmem helpers for KUnit testing

[Gabriele Paoloni <gpaoloni@redhat.com>]

From: Alessandro Carminati <acarmina@redhat.com>

Refactor selected /dev/mem helpers to allow access from KUnit tests.

This change:
  - makes page_is_allowed(), read_mem(), and write_mem() visible when
    KUnit is enabled
  - moves shared declarations into a new internal header
  - preserves existing behavior for non-KUnit builds

Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
---
 drivers/char/mem.c | 15 +++++++++++----
 drivers/char/mem.h | 17 +++++++++++++++++
 2 files changed, 28 insertions(+), 4 deletions(-)
 create mode 100644 drivers/char/mem.h

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 9aa589ea2ef5..f1d0a2c11819 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -31,6 +31,10 @@
 #include <linux/uaccess.h>
 #include <linux/security.h>
 
+#include <kunit/visibility.h>
+
+#include "mem.h"
+
 #define DEVMEM_MINOR	1
 #define DEVPORT_MINOR	4
 
@@ -57,16 +61,17 @@ static inline int valid_mmap_phys_addr_range(unsigned long pfn, size_t size)
 #endif
 
 #ifdef CONFIG_STRICT_DEVMEM
-static inline int page_is_allowed(unsigned long pfn)
+INLINE_VISIBLE_IF_KUNIT int page_is_allowed(unsigned long pfn)
 {
 	return devmem_is_allowed(pfn);
 }
 #else
-static inline int page_is_allowed(unsigned long pfn)
+INLINE_VISIBLE_IF_KUNIT int page_is_allowed(unsigned long pfn)
 {
 	return 1;
 }
 #endif
+EXPORT_SYMBOL_IF_KUNIT(page_is_allowed);
 
 static inline bool should_stop_iteration(void)
 {
@@ -104,7 +109,7 @@ static inline bool should_stop_iteration(void)
  * * %-ENOMEM - out of memory error for auxiliary kernel buffers supporting
  *   the operation of copying content from the physical pages
  */
-static ssize_t read_mem(struct file *file, char __user *buf,
+VISIBLE_IF_KUNIT ssize_t read_mem(struct file *file, char __user *buf,
 			size_t count, loff_t *ppos)
 {
 	phys_addr_t p = *ppos;
@@ -190,6 +195,7 @@ static ssize_t read_mem(struct file *file, char __user *buf,
 	kfree(bounce);
 	return err;
 }
+EXPORT_SYMBOL_IF_KUNIT(read_mem);
 
 /**
  * write_mem - write to physical memory (/dev/mem).
@@ -234,7 +240,7 @@ static ssize_t read_mem(struct file *file, char __user *buf,
  *   be copied from user-space
  * * %-EPERM - access to any of the required pages is not allowed
  */
-static ssize_t write_mem(struct file *file, const char __user *buf,
+VISIBLE_IF_KUNIT ssize_t write_mem(struct file *file, const char __user *buf,
 			 size_t count, loff_t *ppos)
 {
 	phys_addr_t p = *ppos;
@@ -306,6 +312,7 @@ static ssize_t write_mem(struct file *file, const char __user *buf,
 	*ppos += written;
 	return written;
 }
+EXPORT_SYMBOL_IF_KUNIT(write_mem);
 
 int __weak phys_mem_access_prot_allowed(struct file *file,
 	unsigned long pfn, unsigned long size, pgprot_t *vma_prot)
diff --git a/drivers/char/mem.h b/drivers/char/mem.h
new file mode 100644
index 000000000000..dfeb5f1d5ec8
--- /dev/null
+++ b/drivers/char/mem.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2025 Red Hat inc Alessandro Carminati <acarmina@redhat.com>
+ */
+
+#ifndef _LINUX_CHAR_MEM_H
+#define _LINUX_CHAR_MEM_H
+
+#if IS_ENABLED(CONFIG_KUNIT)
+ssize_t read_mem(struct file *file, char __user *buf,
+			size_t count, loff_t *ppos);
+ssize_t write_mem(struct file *file, const char __user *buf,
+			 size_t count, loff_t *ppos);
+int page_is_allowed(unsigned long pfn);
+#endif
+
+#endif /* _LINUX_CHAR_MEM_H */
-- 
2.48.1