[PATCH v2 1/3] ARM: Introduce atomic MMIO clear/set

[Ezequiel Garcia]

Some SoC 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
with clear-set semantics.

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.

---
Based on a similar approach suggested by Russel King:
http://archive.arm.linux.org.uk/lurker/message/20130618.113606.d7d4fe4b.en.html

Changes from v1:

* Added an io barrier iowmb() as suggested by Will Deacon,
  to ensure the writel gets completed before the spin_unlock().

Signed-off-by: Ezequiel Garcia <ezequiel.garcia at free-electrons.com>
---
 arch/arm/include/asm/io.h |  5 +++++
 arch/arm/kernel/io.c      | 13 +++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/arch/arm/include/asm/io.h b/arch/arm/include/asm/io.h
index d070741..3cea1f0 100644
--- a/arch/arm/include/asm/io.h
+++ b/arch/arm/include/asm/io.h
@@ -36,6 +36,11 @@
 #define isa_bus_to_virt phys_to_virt
 
 /*
+ * Atomic MMIO-wide IO clear/set
+ */
+extern void atomic_io_clear_set(u32 clear, u32 set, void __iomem *reg);
+
+/*
  * Generic IO read/write.  These perform native-endian accesses.  Note
  * that some architectures will want to re-define __raw_{read,write}w.
  */
diff --git a/arch/arm/kernel/io.c b/arch/arm/kernel/io.c
index dcd5b4d..b2a53a3 100644
--- a/arch/arm/kernel/io.c
+++ b/arch/arm/kernel/io.c
@@ -1,6 +1,19 @@
 #include <linux/export.h>
 #include <linux/types.h>
 #include <linux/io.h>
+#include <linux/spinlock.h>
+
+static DEFINE_SPINLOCK(__io_lock);
+
+void atomic_io_clear_set(u32 clear, u32 set, void __iomem *reg)
+{
+	spin_lock(&__io_lock);
+	writel((readl(reg) & ~clear) | set, reg);
+	/* ensure the write get done before unlocking */
+	__iowmb();
+	spin_unlock(&__io_lock);
+}
+EXPORT_SYMBOL(atomic_io_clear_set);
 
 /*
  * Copy data from IO memory space to "real" memory space.
-- 
1.8.1.5