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

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Message-ID: <20170921150554.0000273b@huawei.com>
Date:   Thu, 21 Sep 2017 15:05:54 +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 Thu, 21 Sep 2017 14:59:22 +0100
Jonathan Cameron <Jonathan.Cameron@...wei.com> wrote:

> 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>
>

Need more coffee. See below.
 
> > ---
> >  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);

Only free when you actually allocated it.  Seems to me like you need
to check if (!(msg->flags & I2C_M_DMA_SAFE)) before kfree.

Otherwise the logic to do this will be needed in every driver
which will get irritating fast.


> > +}
> > +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  
> 
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