[PATCH] mtd: rawnand: denali: fix a race condition when DMA is kicked

[Masahiro Yamada]

I thought the read-back of the DMA_ENABLE register was unnecessary
(at least it is working on my boards), then deleted it in commit
586a2c52909d ("mtd: nand: denali: squash denali_enable_dma() helper
into caller").  Sorry, I was wrong - it caused a timing issue on
Cyclone5 SoCFPGAs.

Revive the register read-back, commenting why this is necessary.

Fixes: 586a2c52909d ("mtd: nand: denali: squash denali_enable_dma() helper into caller")
Reported-by: Steffen Trumtrar <s.trumtrar@xxxxxxxxxxxxxx>
Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
---

 drivers/mtd/nand/raw/denali.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/mtd/nand/raw/denali.c b/drivers/mtd/nand/raw/denali.c
index 67b2065..b864b93 100644
--- a/drivers/mtd/nand/raw/denali.c
+++ b/drivers/mtd/nand/raw/denali.c
@@ -596,6 +596,12 @@ static int denali_dma_xfer(struct denali_nand_info *denali, void *buf,
 	}
 
 	iowrite32(DMA_ENABLE__FLAG, denali->reg + DMA_ENABLE);
+	/*
+	 * The ->setup_dma() hook kicks DMA by using the data/command
+	 * interface, which belongs to a different AXI port from the
+	 * register interface.  Read back the register to avoid a race.
+	 */
+	ioread32(denali->reg + DMA_ENABLE);
 
 	denali_reset_irq(denali);
 	denali->setup_dma(denali, dma_addr, page, write);
-- 
2.7.4