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, 45 insertions, 0 deletions

diff --git a/include/uapi/asm-generic/sembuf.h b/include/uapi/asm-generic/sembuf.h
new file mode 100644
index 000000000..f54e48fc9
--- /dev/null
+++ b/include/uapi/asm-generic/sembuf.h
@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef __ASM_GENERIC_SEMBUF_H
+#define __ASM_GENERIC_SEMBUF_H
+
+#include <asm/bitsperlong.h>
+#include <asm/ipcbuf.h>
+
+/*
+ * The semid64_ds structure for most architectures (though it came from x86_32
+ * originally). Note extra padding because this structure is passed back and
+ * forth between kernel and user space.
+ *
+ * semid64_ds was originally meant to be architecture specific, but
+ * everyone just ended up making identical copies without specific
+ * optimizations, so we may just as well all use the same one.
+ *
+ * 64 bit architectures use a 64-bit long time field here, while
+ * 32 bit architectures have a pair of unsigned long values.
+ *
+ * On big-endian systems, the padding is in the wrong place for
+ * historic reasons, so user space has to reconstruct a time_t
+ * value using
+ *
+ * user_semid_ds.sem_otime = kernel_semid64_ds.sem_otime +
+ *		((long long)kernel_semid64_ds.sem_otime_high << 32)
+ *
+ * Pad space is left for 2 miscellaneous 32-bit values
+ */
+struct semid64_ds {
+	struct ipc64_perm sem_perm;	/* permissions .. see ipc.h */
+#if __BITS_PER_LONG == 64
+	long		sem_otime;	/* last semop time */
+	long		sem_ctime;	/* last change time */
+#else
+	unsigned long	sem_otime;	/* last semop time */
+	unsigned long	sem_otime_high;
+	unsigned long	sem_ctime;	/* last change time */
+	unsigned long	sem_ctime_high;
+#endif
+	unsigned long	sem_nsems;	/* no. of semaphores in array */
+	unsigned long	__unused3;
+	unsigned long	__unused4;
+};
+
+#endif /* __ASM_GENERIC_SEMBUF_H */