MIPS: sync after cacheflush

On processors with deep write buffers, it is likely that many cycles will pass between a CACHE instruction and the time the data actually gets written out to DRAM. Add a SYNC instruction to ensure that the buffers get emptied before the flush functions return.

Actual problem seen in the wild:

1) dma_alloc_coherent() allocates cached memory

2) memset() is called to clear the new pages

3) dma_cache_wback_inv() is called to flush the zero data out to memory

4) dma_alloc_coherent() returns an uncached (kseg1) pointer to the freshly allocated pages

5) Caller writes data through the kseg1 pointer

6) Buffered writeback data finally gets flushed out to DRAM

7) Part of caller's data is inexplicably zeroed out

This patch adds SYNC between steps 3 and 4, which fixed the problem.

---
 arch/mips/mm/c-r4k.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

index 6721ee2..05c3de3 100644
@@ -605,6 +605,7 @@ static void r4k_dma_cache_wback_inv(unsigned long addr, unsigned long size)
 			r4k_blast_scache();
 		else
 			blast_scache_range(addr, addr + size);
+		__sync();
 		return;
 	}
 

@@ -621,6 +622,7 @@ static void r4k_dma_cache_wback_inv(unsigned long addr, unsigned long size)
 	}
 
 	bc_wback_inv(addr, size);
+	__sync();
 }
 
 static void r4k_dma_cache_inv(unsigned long addr, unsigned long size)

@@ -648,6 +650,7 @@ static void r4k_dma_cache_inv(unsigned long addr, unsigned long size)
 				 (addr + size - 1) & almask);
 			blast_inv_scache_range(addr, addr + size);
 		}
+		__sync();
 		return;
 	}
 

@@ -664,6 +667,7 @@ static void r4k_dma_cache_inv(unsigned long addr, unsigned long size)
 	}
 
 	bc_inv(addr, size);
+	__sync();
 }
 #endif /* CONFIG_DMA_NONCOHERENT */
 

1.7.0.4

Queued for 2.6.41 but I'm probably going to change my mind and will push this earlier.

Thanks!

Ralf