From 354535845ffb74d8a4827fbdaa493037d0030eea Mon Sep 17 00:00:00 2001 From: Russell King Date: Sat, 14 Apr 2012 18:57:10 +0100 Subject: ARM: OMAP: fix DMA vs memory ordering Using coherent DMA memory with the OMAP DMA engine results in unpredictable behaviour due to memory ordering issues; as things stand, there is no guarantee that data written to coherent DMA memory will be visible to the DMA hardware. This is because the OMAP dma_write() accessor contains no barriers, necessary on ARMv6 and above. The effect of this can be seen in comments in the OMAP serial driver, which incorrectly talks about cache flushing for the coherent DMA stuff. Rather than adding barriers to the accessors, add it in the DMA support code just before we enable DMA, and just after we disable DMA. This avoids having barriers for every DMA register access. Acked-by: Tony Lindgren Acked-by: Santosh Shilimkar Signed-off-by: Russell King --- arch/arm/plat-omap/dma.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'arch') diff --git a/arch/arm/plat-omap/dma.c b/arch/arm/plat-omap/dma.c index ecdb3da0dea9..c58d896cd5c3 100644 --- a/arch/arm/plat-omap/dma.c +++ b/arch/arm/plat-omap/dma.c @@ -916,6 +916,13 @@ void omap_start_dma(int lch) l |= OMAP_DMA_CCR_BUFFERING_DISABLE; l |= OMAP_DMA_CCR_EN; + /* + * As dma_write() uses IO accessors which are weakly ordered, there + * is no guarantee that data in coherent DMA memory will be visible + * to the DMA device. Add a memory barrier here to ensure that any + * such data is visible prior to enabling DMA. + */ + mb(); p->dma_write(l, CCR, lch); dma_chan[lch].flags |= OMAP_DMA_ACTIVE; @@ -965,6 +972,13 @@ void omap_stop_dma(int lch) p->dma_write(l, CCR, lch); } + /* + * Ensure that data transferred by DMA is visible to any access + * after DMA has been disabled. This is important for coherent + * DMA regions. + */ + mb(); + if (!omap_dma_in_1510_mode() && dma_chan[lch].next_lch != -1) { int next_lch, cur_lch = lch; char dma_chan_link_map[dma_lch_count]; -- cgit v1.2.3