linux-kernel - Re: [PATCH] staging: gpib: Fix inadvertent negative shift

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Message-ID: <Z4qZdN3xr_Xs6_99@egonzo>
Date: Fri, 17 Jan 2025 18:55:00 +0100
From: Dave Penkler <dpenkler@...il.com>
To: David Laight <david.laight.linux@...il.com>
Cc: gregkh@...uxfoundation.org, linux-staging@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: gpib: Fix inadvertent negative shift

On Thu, Jan 16, 2025 at 10:40:03PM +0000, David Laight wrote:
> On Thu, 16 Jan 2025 12:00:14 +0100
> Dave Penkler <dpenkler@...il.com> wrote:
> 
> > During the initial checkpatch cleanup, when removing blanks
> > after open parentheses, a minus sign in the argument of a shift
> > operation was deleted by mistake. This transformed a pre-decrement
> > operation into a negation.
> > 
> > The result of a negative shift is undefined and a warning was
> > signalled by sparse.
> > 
> > Restore the pre-decrement operation.
> > 
> > Fixes: 6c52d5e3cde2 ("staging: gpib: Add common include files for GPIB drivers")
> > Signed-off-by: Dave Penkler <dpenkler@...il.com>
> > ---
> >  drivers/staging/gpib/include/amcc5920.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/staging/gpib/include/amcc5920.h b/drivers/staging/gpib/include/amcc5920.h
> > index 766b3799223f..7a88bd282feb 100644
> > --- a/drivers/staging/gpib/include/amcc5920.h
> > +++ b/drivers/staging/gpib/include/amcc5920.h
> > @@ -22,7 +22,7 @@ static const int bits_per_region = 8;
> >  
> >  static inline uint32_t amcc_wait_state_bits(unsigned int region, unsigned int num_wait_states)
> >  {
> > -	return (num_wait_states & 0x7) << (-region * bits_per_region);
> > +	return (num_wait_states & 0x7) << (--region * bits_per_region);
> 
> It would be much better to use (region - 1).
Hi,
You are right. I will send a separate patch to change this instance and
all the others in the file.
cheers,
-dave