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authorRussell King <rmk+kernel@arm.linux.org.uk>2012-04-14 18:57:10 +0100
committerRussell King <rmk+kernel@arm.linux.org.uk>2012-04-23 16:54:21 +0100
commit354535845ffb74d8a4827fbdaa493037d0030eea (patch)
treedb21db97f7672f426d21dc8d0097b8b4c38d02fc /arch/arm/plat-omap
parent754aba436bed2051deaa2af7bdda553df056b6fc (diff)
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 <tony@atomide.com> Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Diffstat (limited to 'arch/arm/plat-omap')
-rw-r--r--arch/arm/plat-omap/dma.c14
1 files changed, 14 insertions, 0 deletions
diff --git a/arch/arm/plat-omap/dma.c b/arch/arm/plat-omap/dma.c
index ecdb3da0dea..c58d896cd5c 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];