ARM: 7161/1: errata: no automatic store buffer drain

This patch implements a workaround for PL310 erratum 769419. On
revisions of the PL310 prior to r3p2, the Store Buffer does not
automatically drain. This can cause normal, non-cacheable writes to be
retained when the memory system is idle, leading to suboptimal I/O
performance for drivers using coherent DMA.

This patch adds an optional wmb() call to the cpu_idle loop. On systems
with an outer cache, this causes an explicit flush of the store buffer.

Cc: stable@vger.kernel.org
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

2 files changed, 15 insertions, 0 deletions

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 44789eff983f..83aa746e71d6 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1352,6 +1352,18 @@ config ARM_ERRATA_764369
           relevant cache maintenance functions and sets a specific bit
           in the diagnostic control register of the SCU.
 
+config PL310_ERRATA_769419
+        bool "PL310 errata: no automatic Store Buffer drain"
+        depends on CACHE_L2X0
+        help
+          On revisions of the PL310 prior to r3p2, the Store Buffer does
+          not automatically drain. This can cause normal, non-cacheable
+          writes to be retained when the memory system is idle, leading
+          to suboptimal I/O performance for drivers using coherent DMA.
+          This option adds a write barrier to the cpu_idle loop so that,
+          on systems with an outer cache, the store buffer is drained
+          explicitly.
+
 endmenu
 
 source "arch/arm/common/Kconfig"
diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index 75316f0dd02a..3d0c6fb74ae4 100644
--- a/arch/arm/kernel/process.c
+++ b/arch/arm/kernel/process.c
@@ -192,6 +192,9 @@ void cpu_idle(void)
 #endif
 
                         local_irq_disable();
+#ifdef CONFIG_PL310_ERRATA_769419
+                        wmb();
+#endif
                         if (hlt_counter) {
                                 local_irq_enable();
                                 cpu_relax();