ARM: 6741/1: errata: pl310 cache sync operation may be faulty

The effect of cache sync operation is to drain the store buffer and
wait for all internal buffers to be empty. In normal conditions, store
buffer is able to merge the normal memory writes within its 32-byte
data buffers.  Due to this erratum present in r3p0, the effect of cache
sync operation on the store buffer still remains when the operation
completes. This means that the store buffer is always asked to drain
and this prevents it from merging any further writes.

This can severely affect performance on the write traffic esp. on
Normal memory NC one.

The proposed workaround is to replace the normal offset of cache sync
operation(0x730) by another offset targeting an unmapped PL310
register 0x740.

Signed-off-by: srinidhi kasagar <srinidhi.kasagar@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

3 files changed, 22 insertions, 0 deletions

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 26d45e5b636b..ba9fc213f344 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -1177,6 +1177,21 @@ config ARM_ERRATA_743622
           visible impact on the overall performance or power consumption of the
           processor.
 
+config ARM_ERRATA_753970
+        bool "ARM errata: cache sync operation may be faulty"
+        depends on CACHE_PL310
+        help
+          This option enables the workaround for the 753970 PL310 (r3p0) erratum.
+
+          Under some condition the effect of cache sync operation on
+          the store buffer still remains when the operation completes.
+          This means that the store buffer is always asked to drain and
+          this prevents it from merging any further writes. The workaround
+          is to replace the normal offset of cache sync operation (0x730)
+          by another offset targeting an unmapped PL310 register 0x740.
+          This has the same effect as the cache sync operation: store buffer
+          drain and waiting for all buffers empty.
+
 endmenu
 
 source "arch/arm/common/Kconfig"
diff --git a/arch/arm/include/asm/hardware/cache-l2x0.h b/arch/arm/include/asm/hardware/cache-l2x0.h
index 5aeec1e1735c..16bd48031583 100644
--- a/arch/arm/include/asm/hardware/cache-l2x0.h
+++ b/arch/arm/include/asm/hardware/cache-l2x0.h
@@ -36,6 +36,7 @@
 #define L2X0_RAW_INTR_STAT              0x21C
 #define L2X0_INTR_CLEAR                 0x220
 #define L2X0_CACHE_SYNC                 0x730
+#define L2X0_DUMMY_REG                  0x740
 #define L2X0_INV_LINE_PA                0x770
 #define L2X0_INV_WAY                    0x77C
 #define L2X0_CLEAN_LINE_PA              0x7B0
diff --git a/arch/arm/mm/cache-l2x0.c b/arch/arm/mm/cache-l2x0.c
index 170c9bb95866..f2ce38e085d2 100644
--- a/arch/arm/mm/cache-l2x0.c
+++ b/arch/arm/mm/cache-l2x0.c
@@ -49,7 +49,13 @@ static inline void cache_wait(void __iomem *reg, unsigned long mask)
 static inline void cache_sync(void)
 {
         void __iomem *base = l2x0_base;
+
+#ifdef CONFIG_ARM_ERRATA_753970
+        /* write to an unmmapped register */
+        writel_relaxed(0, base + L2X0_DUMMY_REG);
+#else
         writel_relaxed(0, base + L2X0_CACHE_SYNC);
+#endif
         cache_wait(base + L2X0_CACHE_SYNC, 1);
 }