ARM: 9480/1: entry: expand comment in __switch_tografted

As per discussion between the developers in the mail thread
linked, expand the comment in __switch_to so that readers
of the code understand what is going on.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

1 files changed, 31 insertions, 0 deletions

diff --git a/include/vdso/page.h b/include/vdso/page.h
new file mode 100644
index 000000000..bc47186c0
--- /dev/null
+++ b/include/vdso/page.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __VDSO_PAGE_H
+#define __VDSO_PAGE_H
+
+#include <uapi/linux/const.h>
+
+/*
+ * PAGE_SHIFT determines the page size.
+ *
+ * Note: This definition is required because PAGE_SHIFT is used
+ * in several places throughout the codebase.
+ */
+#define PAGE_SHIFT      CONFIG_PAGE_SHIFT
+
+#define PAGE_SIZE	(_AC(1,UL) << CONFIG_PAGE_SHIFT)
+
+#if !defined(CONFIG_64BIT)
+/*
+ * Applies only to 32-bit architectures.
+ *
+ * Subtle: (1 << CONFIG_PAGE_SHIFT) is an int, not an unsigned long.
+ * So if we assign PAGE_MASK to a larger type it gets extended the
+ * way we want (i.e. with 1s in the high bits) while masking a
+ * 64-bit value such as phys_addr_t.
+ */
+#define PAGE_MASK	(~((1 << CONFIG_PAGE_SHIFT) - 1))
+#else
+#define PAGE_MASK	(~(PAGE_SIZE - 1))
+#endif
+
+#endif	/* __VDSO_PAGE_H */