[RFC] ARM: l2x0: avoid spinlock for sync op on pl310

Commit Message

Rob Herring Sept. 10, 2012, 7:45 p.m. UTC

From: Rob Herring <[e-mail]>

The sync op is atomic on the pl310, so a spinlock is not needed. It can
be a bottleneck for code paths with register accesses, so remove it.
Removing it gives a 30% improvement to pktgen throughput on highbank.

A similar spinlock removal was originally done by Catalin Marinas[1], but
the spinlock part was dropped in the merged version. It is unclear why
other than it was not a runtime selection. As every readl/writel causes a
outer_sync, the sync function is likely the most critical.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2010-August/024514.html

Signed-off-by: Rob Herring <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
---
 arch/arm/mm/cache-l2x0.c |    6 ++++++
 1 file changed, 6 insertions(+)

Comments

Russell King - ARM Linux Sept. 10, 2012, 11:13 p.m. UTC | #1

On Mon, Sep 10, 2012 at 02:45:34PM -0500, Rob Herring wrote:
> From: Rob Herring <[e-mail]>
> 
> The sync op is atomic on the pl310, so a spinlock is not needed. It can
> be a bottleneck for code paths with register accesses, so remove it.
> Removing it gives a 30% improvement to pktgen throughput on highbank.
> 
> A similar spinlock removal was originally done by Catalin Marinas[1], but
> the spinlock part was dropped in the merged version. It is unclear why
> other than it was not a runtime selection. As every readl/writel causes a
> outer_sync, the sync function is likely the most critical.

See:

http://lists.arm.linux.org.uk/lurker/message/20110215.164340.dc0ec480.en.html

Patch

diff --git a/arch/arm/mm/cache-l2x0.c b/arch/arm/mm/cache-l2x0.c
index 2a8e380..6778238 100644
--- a/arch/arm/mm/cache-l2x0.c
+++ b/arch/arm/mm/cache-l2x0.c
@@ -130,6 +130,11 @@  static void l2x0_cache_sync(void)
 	raw_spin_unlock_irqrestore(&l2x0_lock, flags);
 }
 
+static void pl310_cache_sync(void)
+{
+	cache_sync();
+}
+
 static void __l2x0_flush_all(void)
 {
 	debug_writel(0x03);
@@ -335,6 +340,7 @@  void __init l2x0_init(void __iomem *base, u32 aux_val, u32 aux_mask)
 		sync_reg_offset = L2X0_DUMMY_REG;
 #endif
 		outer_cache.set_debug = pl310_set_debug;
+		outer_cache.sync = pl310_cache_sync;
 		break;
 	case L2X0_CACHE_ID_PART_L210:
 		ways = (aux >> 13) & 0xf;