linux-kernel - Re: [RFC PATCH v5 2/6] i2c: add helpers to ease DMA handling

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Date:   Thu, 21 Sep 2017 14:59:22 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Wolfram Sang <wsa+renesas@...g-engineering.com>
CC:     <linux-i2c@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-renesas-soc@...r.kernel.org>, <linux-iio@...r.kernel.org>,
        <linux-input@...r.kernel.org>, <linux-media@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>
Subject: Re: [RFC PATCH v5 2/6] i2c: add helpers to ease DMA handling

On Wed, 20 Sep 2017 20:59:52 +0200
Wolfram Sang <wsa+renesas@...g-engineering.com> wrote:

> One helper checks if DMA is suitable and optionally creates a bounce
> buffer, if not. The other function returns the bounce buffer and makes
> sure the data is properly copied back to the message.
> 
> Signed-off-by: Wolfram Sang <wsa+renesas@...g-engineering.com>

One minor suggestion for wording. Otherwise looks good to me.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>

> ---
>  drivers/i2c/i2c-core-base.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/i2c.h         |  3 +++
>  2 files changed, 48 insertions(+)
> 
> diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
> index 56e46581b84bdb..925a22794d3ced 100644
> --- a/drivers/i2c/i2c-core-base.c
> +++ b/drivers/i2c/i2c-core-base.c
> @@ -2241,6 +2241,51 @@ void i2c_put_adapter(struct i2c_adapter *adap)
>  }
>  EXPORT_SYMBOL(i2c_put_adapter);
>  
> +/**
> + * i2c_get_dma_safe_msg_buf() - get a DMA safe buffer for the given i2c_msg
> + * @msg: the message to be checked
> + * @threshold: the amount of byte from which using DMA makes sense

the minimum number of bytes for which using DMA makes sense.

> + *
> + * Return: NULL if a DMA safe buffer was not obtained. Use msg->buf with PIO.
> + *
> + *	   Or a valid pointer to be used with DMA. Note that it can either be
> + *	   msg->buf or a bounce buffer. After use, release it by calling
> + *	   i2c_release_dma_safe_msg_buf().
> + *
> + * This function must only be called from process context!
> + */
> +u8 *i2c_get_dma_safe_msg_buf(struct i2c_msg *msg, unsigned int threshold)
> +{
> +	if (msg->len < threshold)
> +		return NULL;
> +
> +	if (msg->flags & I2C_M_DMA_SAFE)
> +		return msg->buf;
> +
> +	if (msg->flags & I2C_M_RD)
> +		return kzalloc(msg->len, GFP_KERNEL);
> +	else
> +		return kmemdup(msg->buf, msg->len, GFP_KERNEL);
> +}
> +EXPORT_SYMBOL_GPL(i2c_get_dma_safe_msg_buf);
> +
> +/**
> + * i2c_release_dma_safe_msg_buf - release DMA safe buffer and sync with i2c_msg
> + * @msg: the message to be synced with
> + * @buf: the buffer obtained from i2c_get_dma_safe_msg_buf(). May be NULL.
> + */
> +void i2c_release_dma_safe_msg_buf(struct i2c_msg *msg, u8 *buf)
> +{
> +	if (!buf || buf == msg->buf)
> +		return;
> +
> +	if (msg->flags & I2C_M_RD)
> +		memcpy(msg->buf, buf, msg->len);
> +
> +	kfree(buf);
> +}
> +EXPORT_SYMBOL_GPL(i2c_release_dma_safe_msg_buf);
> +
>  MODULE_AUTHOR("Simon G. Vogl <simon@...uni-linz.ac.at>");
>  MODULE_DESCRIPTION("I2C-Bus main module");
>  MODULE_LICENSE("GPL");
> diff --git a/include/linux/i2c.h b/include/linux/i2c.h
> index d501d3956f13f0..1e99342f180f45 100644
> --- a/include/linux/i2c.h
> +++ b/include/linux/i2c.h
> @@ -767,6 +767,9 @@ static inline u8 i2c_8bit_addr_from_msg(const struct i2c_msg *msg)
>  	return (msg->addr << 1) | (msg->flags & I2C_M_RD ? 1 : 0);
>  }
>  
> +u8 *i2c_get_dma_safe_msg_buf(struct i2c_msg *msg, unsigned int threshold);
> +void i2c_release_dma_safe_msg_buf(struct i2c_msg *msg, u8 *buf);
> +
>  int i2c_handle_smbus_host_notify(struct i2c_adapter *adap, unsigned short addr);
>  /**
>   * module_i2c_driver() - Helper macro for registering a modular I2C driver