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Date:   Thu, 5 May 2022 08:18:17 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Michael Walle <michael@walle.cc>
Cc:     Codrin Ciubotariu <codrin.ciubotariu@microchip.com>,
        Nicolas Ferre <nicolas.ferre@microchip.com>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        Claudiu Beznea <claudiu.beznea@microchip.com>,
        linux-i2c@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        Wolfram Sang <wsa@kernel.org>
Subject: Re: [PATCH v2] i2c: at91: use dma safe buffers
Message-ID: <YnPquUxSSlJehVgY@dev-arch.thelio-3990X>
References: <20220407150828.202513-1-michael@walle.cc>
 <YnPkfrI4Udl9lMR8@dev-arch.thelio-3990X>
 <54f02090e6621460b66dc9d233d53283@walle.cc>
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On Thu, May 05, 2022 at 05:11:30PM +0200, Michael Walle wrote:
> [+ 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@walle.cc>
> > > Reviewed-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
> > > ---
> > > 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?

I can send a patch, thank you for the input!

Cheers,
Nathan