[PATCH] drm/radeon/rs780: prevent division by zero in refresh rate calculation

[Evgenii Burenchev <evg28bur@yandex.ru>]

drm_mode_vrefresh() may return zero when mode clock is zero even if
htotal and vtotal are non-zero. Current code checks only htotal and
vtotal, allowing refresh_rate to become zero and subsequently causing
division by zero in rs780_program_at().

Fix by adding mode clock validation and fallback to default 60Hz in
rs780_get_pm_mode_parameters(). Add WARN_ON in rs780_program_at() to
catch such cases during development, ensuring safe fallback in all
scenarios.

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

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604300508.yXci8rey-lkp@intel.com/

Signed-off-by: Evgenii Burenchev <evg28bur@yandex.ru>
---
 drivers/gpu/drm/radeon/rs780_dpm.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/radeon/rs780_dpm.c b/drivers/gpu/drm/radeon/rs780_dpm.c
index 64bb4cafb8b5..8713f3fd6268 100644
--- a/drivers/gpu/drm/radeon/rs780_dpm.c
+++ b/drivers/gpu/drm/radeon/rs780_dpm.c
@@ -63,8 +63,11 @@ static void rs780_get_pm_mode_parameters(struct radeon_device *rdev)
 		if (crtc && crtc->enabled) {
 			radeon_crtc = to_radeon_crtc(crtc);
 			pi->crtc_id = radeon_crtc->crtc_id;
-			if (crtc->mode.htotal && crtc->mode.vtotal)
+			if (crtc->mode.htotal && crtc->mode.vtotal) {
 				pi->refresh_rate = drm_mode_vrefresh(&crtc->mode);
+				if (pi->refresh_rate == 0)
+					pi->refresh_rate = 60;
+			}
 			break;
 		}
 	}
@@ -363,6 +366,8 @@ static void rs780_program_at(struct radeon_device *rdev)
 {
 	struct igp_power_info *pi = rs780_get_pi(rdev);
 
+	WARN_ON(pi->refresh_rate == 0);
+
 	WREG32(FVTHROT_TARGET_REG, 30000000 / pi->refresh_rate);
 	WREG32(FVTHROT_CB1, 1000000 * 5 / pi->refresh_rate);
 	WREG32(FVTHROT_CB2, 1000000 * 10 / pi->refresh_rate);

base-commit: a5640267d6d35b89ebe6418da90a952a247215a5
-- 
2.43.0