linux-kernel - Re: [PATCH v1] Input: ads7846: ads7846_get_value

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Date:   Sat, 16 Oct 2021 21:23:47 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Mark Brown <broonie@...nel.org>,
        kernel test robot <lkp@...el.com>, kernel@...gutronix.de,
        linux-kernel@...r.kernel.org, linux-input@...r.kernel.org,
        linux-spi@...r.kernel.org, David Jander <david@...tonic.nl>
Subject: Re: [PATCH v1] Input: ads7846: ads7846_get_value - fix unaligned
 pointer value warning

On Fri, Sep 10, 2021 at 09:51:54AM +0200, Oleksij Rempel wrote:
> Hi Dmitry,
> 
> On Thu, Sep 09, 2021 at 10:04:16PM -0700, Dmitry Torokhov wrote:
> > Hi Oleksij,
> > 
> > On Wed, Jul 07, 2021 at 02:41:15PM +0200, Oleksij Rempel wrote:
> > > Fix warning reported by the kernel test robot:
> > > drivers/input/touchscreen/ads7846.c:705:24: warning: taking address
> > > of packed member 'data' of class or structure 'ads7846_buf' may result
> > > in an unaligned pointer value [-Waddress-of-packed-member]
> > > 
> > > Fixes: 6965eece2a89 ("Input: ads7846 - convert to one message")
> > > Reported-by: kernel test robot <lkp@...el.com>
> > > Signed-off-by: Oleksij Rempel <o.rempel@...gutronix.de>
> > > ---
> > >  drivers/input/touchscreen/ads7846.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/input/touchscreen/ads7846.c b/drivers/input/touchscreen/ads7846.c
> > > index adb4c2230c31..1987dedac94f 100644
> > > --- a/drivers/input/touchscreen/ads7846.c
> > > +++ b/drivers/input/touchscreen/ads7846.c
> > > @@ -702,7 +702,7 @@ static int ads7846_get_value(struct ads7846_buf *buf)
> > >  {
> > >  	int value;
> > >  
> > > -	value = be16_to_cpup(&buf->data);
> > > +	value = get_unaligned_be16(&buf->data);
> > 
> > ds7846_buf is declared as packed so I believe using be16_to_cpu(buf->data)
> > will suffice.
> 
> The pointer to the data is is off by 8 bits, so it is not naturally
> aligned. This is why we need to use function which is dealing with
> unaligned pointers. 

As I mentioned, the structure is packed, so the compiler will take care
of doing unaligned access safely on architectures that can not tolerate
it.

Thanks.

-- 
Dmitry