[PATCH 1/4] libfs: add simple_read_from_iomem()

[Michal Wajdeczko <michal.wajdeczko@intel.com>]

It's similar to simple_read_from_buffer() but instead allows to
copy data from the I/O memory in PAGE_SIZE chunks.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
---
Cc: linux-fsdevel@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
---
 fs/libfs.c         | 50 ++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/fs.h |  3 +++
 2 files changed, 53 insertions(+)

diff --git a/fs/libfs.c b/fs/libfs.c
index 3a6f2cb364f8..be8aa42a2f11 100644
--- a/fs/libfs.c
+++ b/fs/libfs.c
@@ -15,6 +15,7 @@
 #include <linux/mutex.h>
 #include <linux/namei.h>
 #include <linux/exportfs.h>
+#include <linux/io.h>
 #include <linux/iversion.h>
 #include <linux/writeback.h>
 #include <linux/buffer_head.h> /* sync_mapping_buffers */
@@ -1040,6 +1041,55 @@ void simple_release_fs(struct vfsmount **mount, int *count)
 }
 EXPORT_SYMBOL(simple_release_fs);
 
+/**
+ * simple_read_from_iomem - copy data from the I/O memory to user space
+ * @to: the user space buffer to read to
+ * @count: the maximum number of bytes to read
+ * @ppos: the current position in the buffer
+ * @from: the I/O memory to read from
+ * @available: the size of the iomem memory
+ *
+ * The simple_read_from_iomem() function reads up to @count bytes (but no
+ * more than %PAGE_SIZE bytes) from the I/O memory @from at offset @ppos
+ * into the user space address starting at @to.
+ *
+ * Return: On success, the number of bytes read is returned and the offset
+ * @ppos is advanced by this number, or negative value is returned on error.
+ */
+ssize_t simple_read_from_iomem(void __user *to, size_t count, loff_t *ppos,
+			       const volatile void __iomem *from, size_t available)
+{
+	loff_t pos = *ppos;
+	size_t ret;
+	void *buf;
+
+	if (pos < 0)
+		return -EINVAL;
+	if (pos >= available || !count)
+		return 0;
+	if (count > available - pos)
+		count = available - pos;
+	if (count > PAGE_SIZE)
+		count = PAGE_SIZE;
+
+	buf = kmalloc(count, GFP_NOWAIT | __GFP_NOWARN);
+	if (!buf)
+		return -ENOMEM;
+
+	memcpy_fromio(buf, from + pos, count);
+	ret = copy_to_user(to, buf, count);
+
+	kfree(buf);
+
+	if (ret == count)
+		return -EFAULT;
+
+	count -= ret;
+	*ppos = pos + count;
+	return count;
+}
+EXPORT_SYMBOL(simple_read_from_iomem);
+
 /**
  * simple_read_from_buffer - copy data from the buffer to user space
  * @to: the user space buffer to read to
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8dfd53b52744..eb4a7b10a1a0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3331,6 +3331,9 @@ extern ssize_t simple_read_from_buffer(void __user *to, size_t count,
 extern ssize_t simple_write_to_buffer(void *to, size_t available, loff_t *ppos,
 		const void __user *from, size_t count);
 
+ssize_t simple_read_from_iomem(void __user *to, size_t count, loff_t *ppos,
+			       const volatile void __iomem *from, size_t available);
+
 struct offset_ctx {
 	struct maple_tree	mt;
 	unsigned long		next_offset;
-- 
2.43.0