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.

v2: minor coding style fixes (Alex)
In practice this should actually happen as the vbios should be
properly populated.

Fixes: 66229b20 ("drm/radeon/kms: add dpm support for rv7xx (v4)")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/cypress_dpm.c

@@ -557,8 +557,12 @@ static int cypress_populate_mclk_value(struct radeon_device *rdev,
 						     ASIC_INTERNAL_MEMORY_SS, vco_freq)) {
 			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 *
+			u32 clk_s, clk_v;
+
+			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;

drivers/gpu/drm/radeon/ni_dpm.c

@@ -2241,8 +2241,12 @@ static int ni_populate_mclk_value(struct radeon_device *rdev,
 						     ASIC_INTERNAL_MEMORY_SS, vco_freq)) {
 			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 *
+			u32 clk_s, clk_v;
+
+			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;

drivers/gpu/drm/radeon/rv740_dpm.c

@@ -249,8 +249,12 @@ int rv740_populate_mclk_value(struct radeon_device *rdev,
 						     ASIC_INTERNAL_MEMORY_SS, vco_freq)) {
 			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 *
+			u32 clk_s, clk_v;
+
+			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;