i2c-tegra: fix possible race condition after tx

In tegra_i2c_fill_tx_fifo, once we have finished pushing all the bytes
to the I2C hardware controller, the interrupt might happen before we
have updated i2c_dev->msg_buf_remaining at the end of the function.
Then, in tegra_i2c_isr, we will call again tegra_i2c_fill_tx_fifo
triggering weird behaviour. This has been shown to happen under real
conditions.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Acked-by: Rhyland Klein <rklein@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>

1 files changed, 32 insertions, 14 deletions

diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
index 17ded1d2f11d..3c94c4a81a55 100644
--- a/drivers/i2c/busses/i2c-tegra.c
+++ b/drivers/i2c/busses/i2c-tegra.c
@@ -270,14 +270,30 @@ static int tegra_i2c_fill_tx_fifo(struct tegra_i2c_dev *i2c_dev)
 
         /* Rounds down to not include partial word at the end of buf */
         words_to_transfer = buf_remaining / BYTES_PER_FIFO_WORD;
-        if (words_to_transfer > tx_fifo_avail)
-                words_to_transfer = tx_fifo_avail;
 
-        i2c_writesl(i2c_dev, buf, I2C_TX_FIFO, words_to_transfer);
-
-        buf += words_to_transfer * BYTES_PER_FIFO_WORD;
-        buf_remaining -= words_to_transfer * BYTES_PER_FIFO_WORD;
-        tx_fifo_avail -= words_to_transfer;
+        /* It's very common to have < 4 bytes, so optimize that case. */
+        if (words_to_transfer) {
+                if (words_to_transfer > tx_fifo_avail)
+                        words_to_transfer = tx_fifo_avail;
+
+                /*
+                 * Update state before writing to FIFO.  If this casues us
+                 * to finish writing all bytes (AKA buf_remaining goes to 0) we
+                 * have a potential for an interrupt (PACKET_XFER_COMPLETE is
+                 * not maskable).  We need to make sure that the isr sees
+                 * buf_remaining as 0 and doesn't call us back re-entrantly.
+                 */
+                buf_remaining -= words_to_transfer * BYTES_PER_FIFO_WORD;
+                tx_fifo_avail -= words_to_transfer;
+                i2c_dev->msg_buf_remaining = buf_remaining;
+                i2c_dev->msg_buf = buf +
+                        words_to_transfer * BYTES_PER_FIFO_WORD;
+                barrier();
+
+                i2c_writesl(i2c_dev, buf, I2C_TX_FIFO, words_to_transfer);
+
+                buf += words_to_transfer * BYTES_PER_FIFO_WORD;
+        }
 
         /*
          * If there is a partial word at the end of buf, handle it manually to
@@ -287,14 +303,15 @@ static int tegra_i2c_fill_tx_fifo(struct tegra_i2c_dev *i2c_dev)
         if (tx_fifo_avail > 0 && buf_remaining > 0) {
                 BUG_ON(buf_remaining > 3);
                 memcpy(&val, buf, buf_remaining);
+
+                /* Again update before writing to FIFO to make sure isr sees. */
+                i2c_dev->msg_buf_remaining = 0;
+                i2c_dev->msg_buf = NULL;
+                barrier();
+
                 i2c_writel(i2c_dev, val, I2C_TX_FIFO);
-                buf_remaining = 0;
-                tx_fifo_avail--;
         }
 
-        BUG_ON(tx_fifo_avail > 0 && buf_remaining > 0);
-        i2c_dev->msg_buf_remaining = buf_remaining;
-        i2c_dev->msg_buf = buf;
         return 0;
 }
 
@@ -411,9 +428,10 @@ static irqreturn_t tegra_i2c_isr(int irq, void *dev_id)
                         tegra_i2c_mask_irq(i2c_dev, I2C_INT_TX_FIFO_DATA_REQ);
         }
 
-        if ((status & I2C_INT_PACKET_XFER_COMPLETE) &&
-                        !i2c_dev->msg_buf_remaining)
+        if (status & I2C_INT_PACKET_XFER_COMPLETE) {
+                BUG_ON(i2c_dev->msg_buf_remaining);
                 complete(&i2c_dev->msg_complete);
+        }
 
         i2c_writel(i2c_dev, status, I2C_INT_STATUS);
         if (i2c_dev->is_dvc)