linux-kernel - Re: [PATCH v2 2/4] i2c: imx: Fix race condition in dma read

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Message-ID: <20180724075707.7wrqltjj54gjrl4y@pengutronix.de>
Date:   Tue, 24 Jul 2018 09:57:07 +0200
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Esben Haabendal <esben.haabendal@...il.com>
Cc:     linux-i2c@...r.kernel.org,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Yuan Yao <yao.yuan@...escale.com>,
        Esben Haabendal <eha@...f.com>,
        Fabio Estevam <fabio.estevam@....com>,
        Lucas Stach <l.stach@...gutronix.de>,
        Phil Reid <preid@...ctromag.com.au>,
        Clemens Gruber <clemens.gruber@...ruber.com>,
        Peter Rosin <peda@...ntia.se>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/4] i2c: imx: Fix race condition in dma read

On Mon, Jul 09, 2018 at 11:43:02AM +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 | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-imx.c b/drivers/i2c/busses/i2c-imx.c
> index 39cfd98c7b23..d86f152176a4 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;
> @@ -810,6 +807,11 @@ 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;
>  	imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
> +	if (i2c_imx->dma && msgs->len >= DMA_THRESHOLD && !block_data) {
> +		temp = imx_i2c_read_reg(i2c_imx, IMX_I2C_I2CR);
> +		temp |= I2CR_DMAEN;
> +		imx_i2c_write_reg(temp, i2c_imx, IMX_I2C_I2CR);
> +	}

Does this need to be a separate write to the I2CR register? Just before
the if there is temp written to this register, so probably this can be
changed to just:

	if (i2c_imx->dma && msgs->len >= DMA_THRESHOLD && !block_data)
		temp |= I2CR_DMAEN;

when moved before up one line.

I don't find documentation for the LS processors where this
register is described though (and the imx family doesn't seem to support
DMA for i2c).

Other than that this looks reasonable and warrants a

	Fixes: ce1a78840ff7 ("i2c: imx: add DMA support for freescale i2c driver")

.

Best regards
Uwe

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