[PATCH 2/2] i2c: designware: Fix corrupted memory seen in the ISR

[Jan Bottorff]

Errors were happening in the ISR that looked like corrupted
memory. This was because memory writes from the core enabling
interrupts were not yet visible to the core running the ISR. The
kernel log would get the message "i2c_designware APMC0D0F:00:
controller timed out" during in-band IPMI SSIF stress tests.

Add a write barrier before enabling interrupts to assure data written
by the current core is visible to all cores before the interrupt fires.

The ARM Barrier Litmus Tests and Cookbook has an example under
Sending Interrupts and Barriers that matches the usage in this
driver. That document says a DSB barrier is required.

Signed-off-by: Jan Bottorff <janb@xxxxxxxxxxxxxxxxxxxxxx>
---
 drivers/i2c/busses/i2c-designware-master.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/i2c/busses/i2c-designware-master.c b/drivers/i2c/busses/i2c-designware-master.c
index ca1035e010c7..1694ac6bb592 100644
--- a/drivers/i2c/busses/i2c-designware-master.c
+++ b/drivers/i2c/busses/i2c-designware-master.c
@@ -248,6 +248,14 @@ static void i2c_dw_xfer_init(struct dw_i2c_dev *dev)
 	/* Dummy read to avoid the register getting stuck on Bay Trail */
 	regmap_read(dev->map, DW_IC_ENABLE_STATUS, &dummy);
 
+	/*
+	 * To guarantee data written by the current core is visible to
+	 * all cores, a write barrier is required. This needs to be
+	 * before an interrupt causes execution on another core.
+	 * For ARM processors, this needs to be a DSB barrier.
+	 */
+	wmb();
+
 	/* Clear and enable interrupts */
 	regmap_read(dev->map, DW_IC_CLR_INTR, &dummy);
 	regmap_write(dev->map, DW_IC_INTR_MASK, DW_IC_INTR_MASTER_MASK);
-- 
2.41.0