Re: [PATCH v3 1/4] i2c: add helpers to ease DMA handling

[Niklas Söderlund]

Hi Wolfram,

On 2017-07-18 12:23:36 +0200, Wolfram Sang 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@xxxxxxxxxxxxxxxxxxxx>

Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@xxxxxxxxxxxx>

> ---
> Changes since v2:
> 
> * rebased to v4.13-rc1
> * helper functions are not inlined anymore but moved to i2c core
> * __must_check has been added to the buffer check helper
> * the release function has been renamed to contain 'dma' as well
> 
>  drivers/i2c/i2c-core-base.c | 68 +++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/i2c.h         |  5 ++++
>  2 files changed, 73 insertions(+)
> 
> diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
> index c89dac7fd2e7b7..7326a9d2e4eb69 100644
> --- a/drivers/i2c/i2c-core-base.c
> +++ b/drivers/i2c/i2c-core-base.c
> @@ -34,6 +34,7 @@
>  #include <linux/irqflags.h>
>  #include <linux/jump_label.h>
>  #include <linux/kernel.h>
> +#include <linux/mm.h>
>  #include <linux/module.h>
>  #include <linux/mutex.h>
>  #include <linux/of_device.h>
> @@ -44,6 +45,7 @@
>  #include <linux/pm_wakeirq.h>
>  #include <linux/property.h>
>  #include <linux/rwsem.h>
> +#include <linux/sched/task_stack.h>
>  #include <linux/slab.h>
>  
>  #include "i2c-core.h"
> @@ -2240,6 +2242,72 @@ void i2c_put_adapter(struct i2c_adapter *adap)
>  }
>  EXPORT_SYMBOL(i2c_put_adapter);
>  
> +/**
> + * i2c_check_msg_for_dma() - check if a message is suitable for DMA
> + * @msg: the message to be checked
> + * @threshold: the amount of byte from which using DMA makes sense
> + * @ptr_for_bounce_buf: if not NULL, a bounce buffer will be attached to this
> + *			ptr, if needed. The bounce buffer must be freed by the
> + *			caller using i2c_release_dma_bounce_buf().
> + *
> + * Return: -ERANGE if message is smaller than threshold
> + *	   -EFAULT if message buffer is not DMA capable and no bounce buffer
> + *		   was requested
> + *	   -ENOMEM if a bounce buffer could not be created
> + *	   0 if message is suitable for DMA
> + *
> + * The return value must be checked.
> + *
> + * Note: This function should only be called from process context! It uses
> + * helper functions which work on the 'current' task.
> + */
> +int i2c_check_msg_for_dma(struct i2c_msg *msg, unsigned int threshold,
> +					u8 **ptr_for_bounce_buf)
> +{
> +	if (ptr_for_bounce_buf)
> +		*ptr_for_bounce_buf = NULL;
> +
> +	if (msg->len < threshold)
> +		return -ERANGE;
> +
> +	if (!virt_addr_valid(msg->buf) || object_is_on_stack(msg->buf)) {
> +		pr_debug("msg buffer to 0x%04x is not DMA safe%s\n", msg->addr,
> +			 ptr_for_bounce_buf ? ", trying bounce buffer" : "");
> +		if (ptr_for_bounce_buf) {
> +			if (msg->flags & I2C_M_RD)
> +				*ptr_for_bounce_buf = kzalloc(msg->len, GFP_KERNEL);
> +			else
> +				*ptr_for_bounce_buf = kmemdup(msg->buf, msg->len,
> +							      GFP_KERNEL);
> +			if (!*ptr_for_bounce_buf)
> +				return -ENOMEM;
> +		} else {
> +			return -EFAULT;
> +		}
> +	}
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(i2c_check_msg_for_dma);
> +
> +/**
> + * i2c_release_bounce_buf - copy data back from bounce buffer and release it
> + * @msg: the message to be copied back to
> + * @bounce_buf: the bounce buffer obtained from i2c_check_msg_for_dma().
> + *		May be NULL.
> + */
> +void i2c_release_dma_bounce_buf(struct i2c_msg *msg, u8 *bounce_buf)
> +{
> +	if (!bounce_buf)
> +		return;
> +
> +	if (msg->flags & I2C_M_RD)
> +		memcpy(msg->buf, bounce_buf, msg->len);
> +
> +	kfree(bounce_buf);
> +}
> +EXPORT_SYMBOL_GPL(i2c_release_bounce_buf);
> +
>  MODULE_AUTHOR("Simon G. Vogl <simon@xxxxxxxxxxxxxxxxx>");
>  MODULE_DESCRIPTION("I2C-Bus main module");
>  MODULE_LICENSE("GPL");
> diff --git a/include/linux/i2c.h b/include/linux/i2c.h
> index 00ca5b86a753f8..ac02287b6c0d8f 100644
> --- a/include/linux/i2c.h
> +++ b/include/linux/i2c.h
> @@ -766,6 +766,11 @@ static inline u8 i2c_8bit_addr_from_msg(const struct i2c_msg *msg)
>  	return (msg->addr << 1) | (msg->flags & I2C_M_RD ? 1 : 0);
>  }
>  
> +int __must_check i2c_check_msg_for_dma(struct i2c_msg *msg, unsigned int threshold,
> +					u8 **ptr_for_bounce_buf);
> +
> +void i2c_release_dma_bounce_buf(struct i2c_msg *msg, u8 *bounce_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
> -- 
> 2.11.0
> 

-- 
Regards,
Niklas Söderlund