[PATCH xf86-input-synaptics 2/2] Ensure history delta computations are signed

[Peter Hutterer]

On Wed, Feb 08, 2012 at 09:10:51PM +0100, Chase Douglas wrote:
> On 02/08/2012 03:54 PM, Peter Hutterer wrote:
> > On Tue, Feb 07, 2012 at 01:07:08PM -0800, Chase Douglas wrote:
> >> Signed-off-by: Chase Douglas <chase.douglas at canonical.com>
> >> ---
> >>  src/synaptics.c |    2 +-
> >>  1 files changed, 1 insertions(+), 1 deletions(-)
> >>
> >> diff --git a/src/synaptics.c b/src/synaptics.c
> >> index a4c1e5a..106b5ee 100644
> >> --- a/src/synaptics.c
> >> +++ b/src/synaptics.c
> >> @@ -1791,7 +1791,7 @@ HandleTapProcessing(SynapticsPrivate *priv, struct SynapticsHwState *hw,
> >>  }
> >>  
> >>  #define HIST(a) (priv->move_hist[((priv->hist_index - (a) + SYNAPTICS_MOVE_HISTORY) % SYNAPTICS_MOVE_HISTORY)])
> >> -#define HIST_DELTA(a, b, e) ((HIST((a)).e) - (HIST((b)).e))
> >> +#define HIST_DELTA(a, b, e) ((int)((HIST((a)).e) - (HIST((b)).e)))
> > 
> > this only casts the result to a signed int, the actual computation is still
> > undefined for b > a. With the revert, HIST_DELTA is only used in one place
> > so we could either do it there or just write a helper function like this
> 
> Why is it undefined? We can have a negative time difference, which is
> what the calculation was assuming, IIUC.

yeah, but IIRC (unsigned int)a - (unsigned int)b is undefined for b > a.

Cheers,
  Peter