* Josh Poimboeuf <jpoim...@redhat.com> wrote:
> On Wed, Mar 01, 2017 at 11:42:54PM +0100, Arnd Bergmann wrote:
> > On Wed, Mar 1, 2017 at 5:53 PM, Josh Poimboeuf <jpoim...@redhat.com> wrote:
> > > On Wed, Mar 01, 2017 at 04:27:29PM +0100, Arnd Bergmann wrote:
> >
> > > I see no apparent reason for the ud2.
> >
> > It's the possible division by zero. This change would avoid the ud2:
> >
> > diff --git a/drivers/i2c/busses/i2c-img-scb.c
> > b/drivers/i2c/busses/i2c-img-scb.c
> > index db8e8b40569d..a2b09c518225 100644
> > --- a/drivers/i2c/busses/i2c-img-scb.c
> > +++ b/drivers/i2c/busses/i2c-img-scb.c
> > @@ -1196,6 +1196,8 @@ static int img_i2c_init(struct img_i2c *i2c)
> > clk_khz /= prescale;
> >
> > /* Setup the clock increment value */
> > + if (clk_khz < 1)
> > + clk_khz = 1;
> > inc = (256 * 16 * bitrate_khz) / clk_khz;
> >
> > /*
>
> Ok, I see what gcc is doing.
>
> clk_khz = clk_get_rate(i2c->scb_clk) / 1000;
> ...
> inc = (256 * 16 * bitrate_khz) / clk_khz;
>
> Because CONFIG_HAVE_CLK isn't set, clk_get_rate() returns 0, which means
> clk_khz is always zero, so the last statement *always* results in a
> divide-by-zero. So that looks like a bug in the code.
>
> However, I'm baffled by how gcc handles it. Instead of:
>
> a) reporting a compile-time warning/error; or
>
> b) letting the #DE (divide error) exception happen;
>
> it inserts a 'ud2', resulting in a #UD (invalid opcode). Why?!?
Well, technically an invalid opcode is shorter code than generating an
(integer)
division by zero exception, right?
Thanks,
Ingo
