lkml.org 
[lkml]   [2017]   [Mar]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings
On Wed, Mar 01, 2017 at 11:42:54PM +0100, Arnd Bergmann wrote:
> On Wed, Mar 1, 2017 at 5:53 PM, Josh Poimboeuf <jpoimboe@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?!?

--
Josh

\
 
 \ /
  Last update: 2017-03-02 02:11    [W:0.910 / U:0.664 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site