linux-kernel - Re: [PATCH v3 1/3] i2c: imx: Fix race condition in dma read

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Message-ID: <20180816082717.tnoslzw4hwuanvxp@pengutronix.de>
Date:   Thu, 16 Aug 2018 10:27:17 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Esben Haabendal <esben.haabendal@...il.com>
Cc:     linux-i2c@...r.kernel.org, Esben Haabendal <eha@...f.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Lucas Stach <l.stach@...gutronix.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Peter Rosin <peda@...ntia.se>,
        Wei Jinhua <wei.jinhua1@....com.cn>,
        Phil Reid <preid@...ctromag.com.au>,
        Fabio Estevam <fabio.estevam@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/3] i2c: imx: Fix race condition in dma read

On Thu, Aug 09, 2018 at 02:32:05PM +0200, Esben Haabendal wrote:
> From: Esben Haabendal <eha@...f.com>
> 
> This fixes a race condition, where the DMAEN bit ends up being set after
> I2C slave has transmitted a byte following the dummy read.  When that
> happens, an interrupt is generated instead, and no DMA request is generated
> to kickstart the DMA read, and a timeout happens after DMA_TIMEOUT (1 sec).
> 
> Fixed by setting the DMAEN bit before the dummy read.
> 
> Signed-off-by: Esben Haabendal <eha@...f.com>
> ---
>  drivers/i2c/busses/i2c-imx.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
> index 498c5e891649..f7bd6c21da27 100644
> --- a/drivers/i2c/busses/i2c-imx.c
> +++ b/drivers/i2c/busses/i2c-imx.c
> @@ -668,9 +668,6 @@ static int i2c_imx_dma_read(struct imx_i2c_struct *i2c_imx,
>  	struct imx_i2c_dma *dma = i2c_imx->dma;
>  	struct device *dev = &i2c_imx->adapter.dev;
>  
> -	temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
> -	temp |= I2CR_DMAEN;
> -	imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
>  
>  	dma->chan_using = dma->chan_rx;
>  	dma->dma_transfer_dir = DMA_DEV_TO_MEM;
> @@ -809,6 +806,8 @@ static int i2c_imx_read(struct imx_i2c_struct *i2c_imx, struct i2c_msg *msgs, bo
>  	 */
>  	if ((msgs->len - 1) || block_data)
>  		temp &= ~I2CR_TXAK;
> +	if (i2c_imx->dma && msgs->len >= DMA_THRESHOLD && !block_data)
> +		temp |= I2CR_DMAEN;

Instead of duplicating the if condition I'd do:

	int use_dma = i2c_imx->dma && msgs->len >= DMA_THRESHOLD && !block_data;

and then simplify the two conditions to just "use_dma".

With this changed you can add my Ack.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |