[PATCH v4 1/4] lib: Introduce atomic MMIO modify

[Ezequiel Garcia]

Some platforms have MMIO regions that are shared across orthogonal
subsystems. This commit implements a possible solution for the
thread-safe access of such regions through a spinlock-protected API.

Concurrent access is protected with a single spinlock for the
entire MMIO address space. While this protects shared-registers,
it also serializes access to unrelated/unshared registers.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia at free-electrons.com>
---
 include/linux/io.h |  5 +++++
 lib/Makefile       |  2 +-
 lib/atomicio.c     | 27 +++++++++++++++++++++++++++
 3 files changed, 33 insertions(+), 1 deletion(-)
 create mode 100644 lib/atomicio.c

diff --git a/include/linux/io.h b/include/linux/io.h
index f4f42fa..c331dcb 100644
--- a/include/linux/io.h
+++ b/include/linux/io.h
@@ -101,4 +101,9 @@ static inline void arch_phys_wc_del(int handle)
 #define arch_phys_wc_add arch_phys_wc_add
 #endif
 
+#ifndef __HAVE_ARCH_ATOMIC_IO_MODIFY
+/* Atomic MMIO-wide IO modify */
+extern void atomic_io_modify(void __iomem *reg, u32 mask, u32 set);
+#endif
+
 #endif /* _LINUX_IO_H */
diff --git a/lib/Makefile b/lib/Makefile
index 7baccfd..695d6e2 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -13,7 +13,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
 	 sha1.o md5.o irq_regs.o reciprocal_div.o argv_split.o \
 	 proportions.o flex_proportions.o prio_heap.o ratelimit.o show_mem.o \
 	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
-	 earlycpio.o percpu-refcount.o
+	 earlycpio.o percpu-refcount.o atomicio.o
 
 obj-$(CONFIG_ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS) += usercopy.o
 lib-$(CONFIG_MMU) += ioremap.o
diff --git a/lib/atomicio.c b/lib/atomicio.c
new file mode 100644
index 0000000..1750f9d
--- /dev/null
+++ b/lib/atomicio.c
@@ -0,0 +1,27 @@
+#include <linux/io.h>
+#include <linux/spinlock.h>
+
+#ifndef __HAVE_ARCH_ATOMIC_IO_MODIFY
+/*
+ * Generic atomic MMIO modify.
+ *
+ * Allows thread-safe access to registers shared by unrelated subsystems.
+ * The access is protected by a single MMIO-wide lock.
+ *
+ * Optimized variants can be implemented on a per-architecture basis.
+ */
+static DEFINE_RAW_SPINLOCK(__io_lock);
+void atomic_io_modify(void __iomem *reg, u32 mask, u32 set)
+{
+	unsigned long flags;
+	u32 value;
+
+	raw_spin_lock_irqsave(&__io_lock, flags);
+	value = readl(reg) & ~mask;
+	value |= (set & mask);
+	writel(value, reg);
+	raw_spin_unlock_irqrestore(&__io_lock, flags);
+
+}
+EXPORT_SYMBOL(atomic_io_modify);
+#endif
-- 
1.8.1.5