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Subject: Re: [PATCH V9] i2c: tegra: remove BUG() macro
To:     Laxman Dewangan <ldewangan@nvidia.com>,
        Thierry Reding <treding@nvidia.com>,
        Jonathan Hunter <jonathanh@nvidia.com>,
        <linux-i2c@vger.kernel.org>, <linux-tegra@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, Peter Rosin <peda@axentia.se>,
        Wolfram Sang <wsa@the-dreams.de>,
        Dmitry Osipenko <digetx@gmail.com>
CC:     Shardar Mohammed <smohammed@nvidia.com>,
        Sowjanya Komatineni <skomatineni@nvidia.com>,
        Mantravadi Karthik <mkarthik@nvidia.com>,
        "Thierry Reding" <thierry.reding@gmail.com>
References: <1560856182-26072-1-git-send-email-bbiswas@nvidia.com>
From:   Bitan Biswas <bbiswas@nvidia.com>
Message-ID: <a2a94eb5-c3c3-24bb-e7a6-0d02f2546f94@nvidia.com>
Date:   Wed, 19 Jun 2019 22:00:51 -0700
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On 6/18/19 4:09 AM, Bitan Biswas wrote:
> The usage of BUG() macro is generally discouraged in kernel, unless
> it's a problem that results in a physical damage or loss of data.
> This patch removes unnecessary BUG() macros and replaces the rest
> with warning.
> 
> Signed-off-by: Bitan Biswas <bbiswas@nvidia.com>
> ---
>   drivers/i2c/busses/i2c-tegra.c | 47 +++++++++++++++++++++++++++++++++++-------
>   1 file changed, 39 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
> index 4dfb4c1..e9ff96d 100644
> --- a/drivers/i2c/busses/i2c-tegra.c
> +++ b/drivers/i2c/busses/i2c-tegra.c
> @@ -73,6 +73,7 @@
>   #define I2C_ERR_NO_ACK				BIT(0)
>   #define I2C_ERR_ARBITRATION_LOST		BIT(1)
>   #define I2C_ERR_UNKNOWN_INTERRUPT		BIT(2)
> +#define I2C_ERR_RX_BUFFER_OVERFLOW		BIT(3)
>   
>   #define PACKET_HEADER0_HEADER_SIZE_SHIFT	28
>   #define PACKET_HEADER0_PACKET_ID_SHIFT		16
> @@ -489,6 +490,13 @@ static int tegra_i2c_empty_rx_fifo(struct tegra_i2c_dev *i2c_dev)
>   	size_t buf_remaining = i2c_dev->msg_buf_remaining;
>   	int words_to_transfer;
>   
> +	/*
> +	 * Catch overflow due to message fully sent
> +	 * before the check for RX FIFO availability.
> +	 */
> +	if (WARN_ON_ONCE(!(i2c_dev->msg_buf_remaining)))
> +		return -EINVAL;
> +
>   	if (i2c_dev->hw->has_mst_fifo) {
>   		val = i2c_readl(i2c_dev, I2C_MST_FIFO_STATUS);
>   		rx_fifo_avail = (val & I2C_MST_FIFO_STATUS_RX_MASK) >>
> @@ -515,7 +523,11 @@ static int tegra_i2c_empty_rx_fifo(struct tegra_i2c_dev *i2c_dev)
>   	 * prevent overwriting past the end of buf
>   	 */
>   	if (rx_fifo_avail > 0 && buf_remaining > 0) {
> -		BUG_ON(buf_remaining > 3);
> +		/*
> +		 * buf_remaining > 3 check not needed as rx_fifo_avail == 0
> +		 * when (words_to_transfer was > rx_fifo_avail) earlier
> +		 * in this function.
> +		 */
>   		val = i2c_readl(i2c_dev, I2C_RX_FIFO);
>   		val = cpu_to_le32(val);
>   		memcpy(buf, &val, buf_remaining);
> @@ -523,7 +535,10 @@ static int tegra_i2c_empty_rx_fifo(struct tegra_i2c_dev *i2c_dev)
>   		rx_fifo_avail--;
>   	}
>   
> -	BUG_ON(rx_fifo_avail > 0 && buf_remaining > 0);
> +	/* RX FIFO must be drained, otherwise it's an Overflow case. */
> +	if (WARN_ON_ONCE(rx_fifo_avail))
> +		return -EINVAL;
> +
>   	i2c_dev->msg_buf_remaining = buf_remaining;
>   	i2c_dev->msg_buf = buf;
>   
> @@ -581,7 +596,11 @@ static int tegra_i2c_fill_tx_fifo(struct tegra_i2c_dev *i2c_dev)
>   	 * boundary and fault.
>   	 */
>   	if (tx_fifo_avail > 0 && buf_remaining > 0) {
> -		BUG_ON(buf_remaining > 3);
> +		/*
> +		 * buf_remaining > 3 check not needed as tx_fifo_avail == 0
> +		 * when (words_to_transfer was > tx_fifo_avail) earlier
> +		 * in this function for non-zero words_to_transfer.
> +		 */
>   		memcpy(&val, buf, buf_remaining);
>   		val = le32_to_cpu(val);
>   
> @@ -847,10 +866,15 @@ static irqreturn_t tegra_i2c_isr(int irq, void *dev_id)
>   
>   	if (!i2c_dev->is_curr_dma_xfer) {
>   		if (i2c_dev->msg_read && (status & I2C_INT_RX_FIFO_DATA_REQ)) {
> -			if (i2c_dev->msg_buf_remaining)
> -				tegra_i2c_empty_rx_fifo(i2c_dev);
> -			else
> -				BUG();
> +			if (tegra_i2c_empty_rx_fifo(i2c_dev)) {
> +				/*
> +				 * Overflow error condition: message fully sent,
> +				 * with no XFER_COMPLETE interrupt but hardware
> +				 * asks to transfer more.
> +				 */
> +				i2c_dev->msg_err |= I2C_ERR_RX_BUFFER_OVERFLOW;
> +				goto err;
> +			}
>   		}
>   
>   		if (!i2c_dev->msg_read && (status & I2C_INT_TX_FIFO_DATA_REQ)) {
> @@ -876,7 +900,14 @@ static irqreturn_t tegra_i2c_isr(int irq, void *dev_id)
>   	if (status & I2C_INT_PACKET_XFER_COMPLETE) {
>   		if (i2c_dev->is_curr_dma_xfer)
>   			i2c_dev->msg_buf_remaining = 0;
> -		BUG_ON(i2c_dev->msg_buf_remaining);
> +		/*
> +		 * Underflow error condition: XFER_COMPLETE before message
> +		 * fully sent.
> +		 */
> +		if (WARN_ON_ONCE(i2c_dev->msg_buf_remaining)) {
> +			i2c_dev->msg_err |= I2C_ERR_UNKNOWN_INTERRUPT;
> +			goto err;
> +		}
>   		complete(&i2c_dev->msg_complete);
>   	}
>   	goto done;
> 

Please get back if there are any further comments regarding this patch.

-regards,
  Bitan