Re: [PATCH v2] i2c: at91: use dma safe buffers

[Michael Walle]

[+ Wolfram]

Am 2022-05-05 16:51, schrieb Nathan Chancellor:
> On Thu, Apr 07, 2022 at 05:08:28PM +0200, Michael Walle wrote:
> > The supplied buffer might be on the stack and we get the following 
> > error
> > message:
> > [    3.312058] at91_i2c e0070600.i2c: rejecting DMA map of vmalloc 
> > memory
> >
> > Use i2c_{get,put}_dma_safe_msg_buf() to get a DMA-able memory region 
> > if
> > necessary.
> >
> > Fixes: 60937b2cdbf9 ("i2c: at91: add dma support")
> > Signed-off-by: Michael Walle <michael@xxxxxxxx>
> > Reviewed-by: Codrin Ciubotariu <codrin.ciubotariu@xxxxxxxxxxxxx>
> > ---
> > changes since v1:
> >  - remove extra empty line
> >  - add fixes tag as suggested by Codrin
> >
> >  drivers/i2c/busses/i2c-at91-master.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/drivers/i2c/busses/i2c-at91-master.c 
> > b/drivers/i2c/busses/i2c-at91-master.c
> > index b0eae94909f4..5eca3b3bb609 100644
> > --- a/drivers/i2c/busses/i2c-at91-master.c
> > +++ b/drivers/i2c/busses/i2c-at91-master.c
> > @@ -656,6 +656,7 @@ static int at91_twi_xfer(struct i2c_adapter *adap, 
> > struct i2c_msg *msg, int num)
> >  	unsigned int_addr_flag = 0;
> >  	struct i2c_msg *m_start = msg;
> >  	bool is_read;
> > +	u8 *dma_buf;
> >
> >  	dev_dbg(&adap->dev, "at91_xfer: processing %d messages:\n", num);
> >
> > @@ -703,7 +704,17 @@ static int at91_twi_xfer(struct i2c_adapter 
> > *adap, struct i2c_msg *msg, int num)
> >  	dev->msg = m_start;
> >  	dev->recv_len_abort = false;
> >
> > +	if (dev->use_dma) {
> > +		dma_buf = i2c_get_dma_safe_msg_buf(m_start, 1);
> > +		if (!dma_buf) {
> > +			ret = -ENOMEM;
> > +			goto out;
> > +		}
> > +		dev->buf = dma_buf;
> > +	}
> > +
> >  	ret = at91_do_twi_transfer(dev);
> > +	i2c_put_dma_safe_msg_buf(dma_buf, m_start, !ret);
> >
> >  	ret = (ret < 0) ? ret : num;
> >  out:
> > --
> > 2.30.2
>
> This change as commit 03fbb903c8bf ("i2c: at91: use dma safe buffers")
> causes the following clang warning:

Ugh..

> drivers/i2c/busses/i2c-at91-master.c:707:6: error: variable 'dma_buf'
> is used uninitialized whenever 'if' condition is false
> [-Werror,-Wsometimes-uninitialized]
>         if (dev->use_dma) {
>             ^~~~~~~~~~~~
> drivers/i2c/busses/i2c-at91-master.c:717:27: note: uninitialized use 
> occurs here
>         i2c_put_dma_safe_msg_buf(dma_buf, m_start, !ret);
>                                  ^~~~~~~
> drivers/i2c/busses/i2c-at91-master.c:707:2: note: remove the 'if' if
> its condition is always true
>         if (dev->use_dma) {
>         ^~~~~~~~~~~~~~~~~~
> drivers/i2c/busses/i2c-at91-master.c:659:13: note: initialize the
> variable 'dma_buf' to silence this warning
>         u8 *dma_buf;
>                    ^
>                     = NULL
> 1 error generated.
>
> Should this variable be initialized or should the call to
> i2c_put_dma_safe_msg_buf() be moved into the if statement?

Initializing dma_buf with NULL should to the trick because
i2c_put_dma_safe_msg_buf() is handling NULL gracefully. No
need for an extra "if (dma_buf)".

Will you take care or should I send a patch?

-michael