drm/radeon: fix possible division-by-zero errors

  Function rv740_get_decoded_reference_divider() may return 0 due to
unpredictable reference divider value calculated in
radeon_atom_get_clock_dividers(). This will lead to
division-by-zero error once that value is used as a divider
in calculating 'clk_s'.
While unlikely, this issue should nonetheless be prevented so add a
sanity check for such cases by testing 'decoded_ref' value against 0.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 66229b200598 ("drm/radeon/kms: add dpm support for rv7xx (v4)")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>

---
 drivers/gpu/drm/radeon/cypress_dpm.c | 7 +++++--
 drivers/gpu/drm/radeon/ni_dpm.c      | 7 +++++--
 drivers/gpu/drm/radeon/rv740_dpm.c   | 7 +++++--
 3 files changed, 15 insertions(+), 6 deletions(-)
  

In practice this should never happen.  Applied with some minor coding
style fixes.

Alex

On Fri, May 19, 2023 at 11:33 AM Nikita Zhandarovich
<n.zhandarovich@fintech.ru> wrote:
>
> Function rv740_get_decoded_reference_divider() may return 0 due to
> unpredictable reference divider value calculated in
> radeon_atom_get_clock_dividers(). This will lead to
> division-by-zero error once that value is used as a divider
> in calculating 'clk_s'.
> While unlikely, this issue should nonetheless be prevented so add a
> sanity check for such cases by testing 'decoded_ref' value against 0.
>
> Found by Linux Verification Center (linuxtesting.org) with static
> analysis tool SVACE.
>
> Fixes: 66229b200598 ("drm/radeon/kms: add dpm support for rv7xx (v4)")
> Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
>
> ---
>  drivers/gpu/drm/radeon/cypress_dpm.c | 7 +++++--
>  drivers/gpu/drm/radeon/ni_dpm.c      | 7 +++++--
>  drivers/gpu/drm/radeon/rv740_dpm.c   | 7 +++++--
>  3 files changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/radeon/cypress_dpm.c b/drivers/gpu/drm/radeon/cypress_dpm.c
> index fdddbbaecbb7..3678b7e384e1 100644
> --- a/drivers/gpu/drm/radeon/cypress_dpm.c
> +++ b/drivers/gpu/drm/radeon/cypress_dpm.c
> @@ -555,10 +555,13 @@ static int cypress_populate_mclk_value(struct radeon_device *rdev,
>
>                 if (radeon_atombios_get_asic_ss_info(rdev, &ss,
>                                                      ASIC_INTERNAL_MEMORY_SS, vco_freq)) {
> +                       u32 clk_s, clk_v;
>                         u32 reference_clock = rdev->clock.mpll.reference_freq;
>                         u32 decoded_ref = rv740_get_decoded_reference_divider(dividers.ref_div);
> -                       u32 clk_s = reference_clock * 5 / (decoded_ref * ss.rate);
> -                       u32 clk_v = ss.percentage *
> +                       if (!decoded_ref)
> +                               return -EINVAL;
> +                       clk_s = reference_clock * 5 / (decoded_ref * ss.rate);
> +                       clk_v = ss.percentage *
>                                 (0x4000 * dividers.whole_fb_div + 0x800 * dividers.frac_fb_div) / (clk_s * 625);
>
>                         mpll_ss1 &= ~CLKV_MASK;
> diff --git a/drivers/gpu/drm/radeon/ni_dpm.c b/drivers/gpu/drm/radeon/ni_dpm.c
> index 672d2239293e..9ce3e5635efc 100644
> --- a/drivers/gpu/drm/radeon/ni_dpm.c
> +++ b/drivers/gpu/drm/radeon/ni_dpm.c
> @@ -2239,10 +2239,13 @@ static int ni_populate_mclk_value(struct radeon_device *rdev,
>
>                 if (radeon_atombios_get_asic_ss_info(rdev, &ss,
>                                                      ASIC_INTERNAL_MEMORY_SS, vco_freq)) {
> +                       u32 clk_s, clk_v;
>                         u32 reference_clock = rdev->clock.mpll.reference_freq;
>                         u32 decoded_ref = rv740_get_decoded_reference_divider(dividers.ref_div);
> -                       u32 clk_s = reference_clock * 5 / (decoded_ref * ss.rate);
> -                       u32 clk_v = ss.percentage *
> +                       if (!decoded_ref)
> +                               return -EINVAL;
> +                       clk_s = reference_clock * 5 / (decoded_ref * ss.rate);
> +                       clk_v = ss.percentage *
>                                 (0x4000 * dividers.whole_fb_div + 0x800 * dividers.frac_fb_div) / (clk_s * 625);
>
>                         mpll_ss1 &= ~CLKV_MASK;
> diff --git a/drivers/gpu/drm/radeon/rv740_dpm.c b/drivers/gpu/drm/radeon/rv740_dpm.c
> index d57a3e1df8d6..ca76efa0f59d 100644
> --- a/drivers/gpu/drm/radeon/rv740_dpm.c
> +++ b/drivers/gpu/drm/radeon/rv740_dpm.c
> @@ -247,10 +247,13 @@ int rv740_populate_mclk_value(struct radeon_device *rdev,
>
>                 if (radeon_atombios_get_asic_ss_info(rdev, &ss,
>                                                      ASIC_INTERNAL_MEMORY_SS, vco_freq)) {
> +                       u32 clk_s, clk_v;
>                         u32 reference_clock = rdev->clock.mpll.reference_freq;
>                         u32 decoded_ref = rv740_get_decoded_reference_divider(dividers.ref_div);
> -                       u32 clk_s = reference_clock * 5 / (decoded_ref * ss.rate);
> -                       u32 clk_v = 0x40000 * ss.percentage *
> +                       if (!decoded_ref)
> +                               return -EINVAL;
> +                       clk_s = reference_clock * 5 / (decoded_ref * ss.rate);
> +                       clk_v = 0x40000 * ss.percentage *
>                                 (dividers.whole_fb_div + (dividers.frac_fb_div / 8)) / (clk_s * 10000);
>
>                         mpll_ss1 &= ~CLKV_MASK;
> --
> 2.25.1
>