[PATCH] drm/amd/display: Possible divide by zero in set_speed()

[Alex Deucher]

Applied.  Thanks!

Alex

On Thu, Jan 30, 2020 at 11:58 PM Dan Carpenter <dan.carpenter at oracle.com> wrote:
>
> If "speed" is zero then we use it as a divisor to find "prescale".  It's
> better to move the check for zero to the very start of the function.
>
> Fixes: 9eeec26a1339 ("drm/amd/display: Refine i2c frequency calculating sequence")
> Signed-off-by: Dan Carpenter <dan.carpenter at oracle.com>
> ---
>  drivers/gpu/drm/amd/display/dc/dce/dce_i2c_hw.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/display/dc/dce/dce_i2c_hw.c b/drivers/gpu/drm/amd/display/dc/dce/dce_i2c_hw.c
> index 066188ba7949..24adec407972 100644
> --- a/drivers/gpu/drm/amd/display/dc/dce/dce_i2c_hw.c
> +++ b/drivers/gpu/drm/amd/display/dc/dce/dce_i2c_hw.c
> @@ -267,6 +267,9 @@ static void set_speed(
>         uint32_t xtal_ref_div = 0;
>         uint32_t prescale = 0;
>
> +       if (speed == 0)
> +               return;
> +
>         REG_GET(MICROSECOND_TIME_BASE_DIV, XTAL_REF_DIV, &xtal_ref_div);
>
>         if (xtal_ref_div == 0)
> @@ -274,17 +277,15 @@ static void set_speed(
>
>         prescale = ((dce_i2c_hw->reference_frequency * 2) / xtal_ref_div) / speed;
>
> -       if (speed) {
> -               if (dce_i2c_hw->masks->DC_I2C_DDC1_START_STOP_TIMING_CNTL)
> -                       REG_UPDATE_N(SPEED, 3,
> -                                    FN(DC_I2C_DDC1_SPEED, DC_I2C_DDC1_PRESCALE), prescale,
> -                                    FN(DC_I2C_DDC1_SPEED, DC_I2C_DDC1_THRESHOLD), 2,
> -                                    FN(DC_I2C_DDC1_SPEED, DC_I2C_DDC1_START_STOP_TIMING_CNTL), speed > 50 ? 2:1);
> -               else
> -                       REG_UPDATE_N(SPEED, 2,
> -                                    FN(DC_I2C_DDC1_SPEED, DC_I2C_DDC1_PRESCALE), prescale,
> -                                    FN(DC_I2C_DDC1_SPEED, DC_I2C_DDC1_THRESHOLD), 2);
> -       }
> +       if (dce_i2c_hw->masks->DC_I2C_DDC1_START_STOP_TIMING_CNTL)
> +               REG_UPDATE_N(SPEED, 3,
> +                            FN(DC_I2C_DDC1_SPEED, DC_I2C_DDC1_PRESCALE), prescale,
> +                            FN(DC_I2C_DDC1_SPEED, DC_I2C_DDC1_THRESHOLD), 2,
> +                            FN(DC_I2C_DDC1_SPEED, DC_I2C_DDC1_START_STOP_TIMING_CNTL), speed > 50 ? 2:1);
> +       else
> +               REG_UPDATE_N(SPEED, 2,
> +                            FN(DC_I2C_DDC1_SPEED, DC_I2C_DDC1_PRESCALE), prescale,
> +                            FN(DC_I2C_DDC1_SPEED, DC_I2C_DDC1_THRESHOLD), 2);
>  }
>
>  static bool setup_engine(
> --
> 2.11.0
>
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