[PATCH v3 1/3] ARM: Introduce atomic MMIO modify

[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.

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.

We add relaxed and non-relaxed variants, by using writel_relaxed and writel,
respectively. The rationale for this is that some users may not require
register write completion but only thread-safe access to a register.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia at free-electrons.com>
---
Based on a similar approach suggested by Russell King:
http://archive.arm.linux.org.uk/lurker/message/20130618.113606.d7d4fe4b.en.html

Changes from v2:
* As suggested by Will Deacon, dropped the iowmb() barrier
  and use relaxed variants instead. See Will's explanation for
  details:
  http://www.spinics.net/lists/arm-kernel/msg268775.html

* Use spin_{}_irqsave/restore to allow irq-context usage
  also suggested by Will Deacon.

* Re-work the API semantics as proposed by Russell King.

Changes from v1:
* Added an io barrier iowmb() as suggested by Will Deacon,
  to ensure the writel gets completed before the spin_unlock().

 arch/arm/include/asm/io.h |  6 ++++++
 arch/arm/kernel/io.c      | 29 +++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/arch/arm/include/asm/io.h b/arch/arm/include/asm/io.h
index d070741..27521e1 100644
--- a/arch/arm/include/asm/io.h
+++ b/arch/arm/include/asm/io.h
@@ -36,6 +36,12 @@
 #define isa_bus_to_virt phys_to_virt
 
 /*
+ * Atomic MMIO-wide IO modify
+ */
+extern void atomic_io_modify(void __iomem *reg, u32 mask, u32 set);
+extern void atomic_io_modify_relaxed(void __iomem *reg, u32 mask, u32 set);
+
+/*
  * 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..651f1ad 100644
--- a/arch/arm/kernel/io.c
+++ b/arch/arm/kernel/io.c
@@ -1,6 +1,35 @@
 #include <linux/export.h>
 #include <linux/types.h>
 #include <linux/io.h>
+#include <linux/spinlock.h>
+
+static DEFINE_SPINLOCK(__io_lock);
+
+void atomic_io_modify_relaxed(void __iomem *reg, u32 mask, u32 set)
+{
+	unsigned long flags;
+	u32 value;
+
+	spin_lock_irqsave(&__io_lock, flags);
+	value = readl_relaxed(reg) & ~mask;
+	value |= (set & mask);
+	writel_relaxed(value, reg);
+	spin_unlock_irqrestore(&__io_lock, flags);
+}
+EXPORT_SYMBOL(atomic_io_modify);
+
+void atomic_io_modify(void __iomem *reg, u32 mask, u32 set)
+{
+	unsigned long flags;
+	u32 value;
+
+	spin_lock_irqsave(&__io_lock, flags);
+	value = readl_relaxed(reg) & ~mask;
+	value |= (set & mask);
+	writel(value, reg);
+	spin_unlock_irqrestore(&__io_lock, flags);
+}
+EXPORT_SYMBOL(atomic_io_modify);
 
 /*
  * Copy data from IO memory space to "real" memory space.
-- 
1.8.1.5