linux-kernel - Re: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings

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Date:   Thu, 2 Mar 2017 23:49:49 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH] [RFC] x86: avoid -mtune=atom for objtool warnings

On Thu, Mar 2, 2017 at 2:03 AM, Josh Poimboeuf <jpoimboe@...hat.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 <jpoimboe@...hat.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?!?

Just FYI, I found another one like this:

0000000000000000 <hibvt_pwm_get_state>:
   0:   e8 00 00 00 00          callq  5 <hibvt_pwm_get_state+0x5>
                        1: R_X86_64_PC32        __fentry__-0x4
   5:   8b 46 10                mov    0x10(%rsi),%eax
   8:   55                      push   %rbp
   9:   48 89 e5                mov    %rsp,%rbp
   c:   c1 e0 05                shl    $0x5,%eax
   f:   48 03 47 48             add    0x48(%rdi),%rax
  13:   8b 00                   mov    (%rax),%eax
  15:   0f 0b                   ud2
  17:   66 0f 1f 84 00 00 00    nopw   0x0(%rax,%rax,1)
  1e:   00 00

static inline unsigned long clk_get_rate(struct clk *clk)
{
        return 0;
}

static void hibvt_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
                                struct pwm_state *state)
{
        struct hibvt_pwm_chip *hi_pwm_chip = to_hibvt_pwm_chip(chip);
        void __iomem *base;
        u32 freq, value;

        freq = div_u64(clk_get_rate(hi_pwm_chip->clk), 1000000);
        base = hi_pwm_chip->base;

        value = readl(base + PWM_CFG0_ADDR(pwm->hwpwm));
        state->period = div_u64(value * 1000, freq);

        value = readl(base + PWM_CFG1_ADDR(pwm->hwpwm));
        state->duty_cycle = div_u64(value * 1000, freq);

        value = readl(base + PWM_CTRL_ADDR(pwm->hwpwm));
        state->enabled = (PWM_ENABLE_MASK & value);
}


    Arnd