Re: [PATCH] i2c: tegra: fix possible race condition after tx

Rhyland Klein
2011-08-11T20:53:36Z

On Thu, 2011-08-11 at 13:19 -0700, Doug Anderson wrote:
> 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>
> ---
> drivers/i2c/busses/i2c-tegra.c | 46 +++++++++++++++++++++++++++------------
> 1 files changed, 32 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
> index 2440b74..9c5fb75 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)

Looks good to me

Acked-by: Rhyland Klein <rklein@nvidia.com>