i2c: mxs: ensure that DMA buffers are safe for DMA

We found that after commit 9c46929e
("ARM: implement THREAD_INFO_IN_TASK for uniprocessor systems"), the
PCF85063 RTC driver stopped working on i.MX28 due to regmap_bulk_read()
reading bogus data into a stack buffer. This is caused by the i2c-mxs
driver using DMA transfers even for messages without the I2C_M_DMA_SAFE
flag, and the aforementioned commit enabling vmapped stacks.

As the MXS I2C controller requires DMA for reads of >4 bytes, DMA can't be
disabled, so the issue is fixed by using i2c_get_dma_safe_msg_buf() to
create a bounce buffer when needed.

Fixes: 9c46929e ("ARM: implement THREAD_INFO_IN_TASK for uniprocessor systems")
Signed-off-by: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/busses/i2c-mxs.c

@@ -171,7 +171,7 @@ static void mxs_i2c_dma_irq_callback(void *param)
 }
 
 static int mxs_i2c_dma_setup_xfer(struct i2c_adapter *adap,
-			struct i2c_msg *msg, uint32_t flags)
+			struct i2c_msg *msg, u8 *buf, uint32_t flags)
 {
 	struct dma_async_tx_descriptor *desc;
 	struct mxs_i2c_dev *i2c = i2c_get_adapdata(adap);
@@ -226,7 +226,7 @@ static int mxs_i2c_dma_setup_xfer(struct i2c_adapter *adap,
 		}
 
 		/* Queue the DMA data transfer. */
-		sg_init_one(&i2c->sg_io[1], msg->buf, msg->len);
+		sg_init_one(&i2c->sg_io[1], buf, msg->len);
 		dma_map_sg(i2c->dev, &i2c->sg_io[1], 1, DMA_FROM_DEVICE);
 		desc = dmaengine_prep_slave_sg(i2c->dmach, &i2c->sg_io[1], 1,
 					DMA_DEV_TO_MEM,
@@ -259,7 +259,7 @@ static int mxs_i2c_dma_setup_xfer(struct i2c_adapter *adap,
 		/* Queue the DMA data transfer. */
 		sg_init_table(i2c->sg_io, 2);
 		sg_set_buf(&i2c->sg_io[0], &i2c->addr_data, 1);
-		sg_set_buf(&i2c->sg_io[1], msg->buf, msg->len);
+		sg_set_buf(&i2c->sg_io[1], buf, msg->len);
 		dma_map_sg(i2c->dev, i2c->sg_io, 2, DMA_TO_DEVICE);
 		desc = dmaengine_prep_slave_sg(i2c->dmach, i2c->sg_io, 2,
 					DMA_MEM_TO_DEV,
@@ -563,6 +563,7 @@ static int mxs_i2c_xfer_msg(struct i2c_adapter *adap, struct i2c_msg *msg,
 	struct mxs_i2c_dev *i2c = i2c_get_adapdata(adap);
 	int ret;
 	int flags;
+	u8 *dma_buf;
 	int use_pio = 0;
 	unsigned long time_left;
 
@@ -588,13 +589,20 @@ static int mxs_i2c_xfer_msg(struct i2c_adapter *adap, struct i2c_msg *msg,
 		if (ret && (ret != -ENXIO))
 			mxs_i2c_reset(i2c);
 	} else {
+		dma_buf = i2c_get_dma_safe_msg_buf(msg, 1);
+		if (!dma_buf)
+			return -ENOMEM;
+
 		reinit_completion(&i2c->cmd_complete);
-		ret = mxs_i2c_dma_setup_xfer(adap, msg, flags);
-		if (ret)
+		ret = mxs_i2c_dma_setup_xfer(adap, msg, dma_buf, flags);
+		if (ret) {
+			i2c_put_dma_safe_msg_buf(dma_buf, msg, false);
 			return ret;
+		}
 
 		time_left = wait_for_completion_timeout(&i2c->cmd_complete,
 						msecs_to_jiffies(1000));
+		i2c_put_dma_safe_msg_buf(dma_buf, msg, true);
 		if (!time_left)
 			goto timeout;