[PATCH] drm/i915/gvt: Prevent divided by zero when calculating refresh rate

[Colin Xu]

To get refresh rate as vblank timer period and keep the precision, the
calculation of rate is multiplied by 1000. However old logic was using:
rate = pixel clock / (h * v / 1000). When the h/v total is invalid, like
all 0, h * v / 1000 will be rounded to 0, which leads to a divided by 0
fault.

0 H/V are already checked above. Instead of divide after divide, refine
the calculation to divide after multiply: "pixel clock * 1000 / (h * v)"
Guest driver should guarantee the correctness of the timing regs' value.

Fixes: 6a4500c7b83f ("drm/i915/gvt: Get accurate vGPU virtual display
refresh rate from vreg")

Reported-by: Zhenyu Wang <zhenyuw at linux.intel.com>
Signed-off-by: Colin Xu <colin.xu at intel.com>
---
 drivers/gpu/drm/i915/gvt/handlers.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gvt/handlers.c b/drivers/gpu/drm/i915/gvt/handlers.c
index 477badfcb258..dda320749c65 100644
--- a/drivers/gpu/drm/i915/gvt/handlers.c
+++ b/drivers/gpu/drm/i915/gvt/handlers.c
@@ -669,8 +669,8 @@ static void vgpu_update_refresh_rate(struct intel_vgpu *vgpu)
 	link_n = vgpu_vreg_t(vgpu, PIPE_LINK_N1(TRANSCODER_A));
 
 	/* Get H/V total from transcoder timing */
-	htotal = (vgpu_vreg_t(vgpu, HTOTAL(TRANSCODER_A)) >> TRANS_HTOTAL_SHIFT) + 1;
-	vtotal = (vgpu_vreg_t(vgpu, VTOTAL(TRANSCODER_A)) >> TRANS_VTOTAL_SHIFT) + 1;
+	htotal = (vgpu_vreg_t(vgpu, HTOTAL(TRANSCODER_A)) >> TRANS_HTOTAL_SHIFT);
+	vtotal = (vgpu_vreg_t(vgpu, VTOTAL(TRANSCODER_A)) >> TRANS_VTOTAL_SHIFT);
 
 	if (dp_br && link_n && htotal && vtotal) {
 		u64 pixel_clk = 0;
@@ -682,7 +682,7 @@ static void vgpu_update_refresh_rate(struct intel_vgpu *vgpu)
 		pixel_clk *= MSEC_PER_SEC;
 
 		/* Calcuate refresh rate by (pixel_clk / (h_total * v_total)) */
-		new_rate = DIV64_U64_ROUND_CLOSEST(pixel_clk, div64_u64(mul_u32_u32(htotal, vtotal), MSEC_PER_SEC));
+		new_rate = DIV64_U64_ROUND_CLOSEST(mul_u64_u32_shr(pixel_clk, MSEC_PER_SEC, 0), mul_u32_u32(htotal + 1, vtotal + 1));
 
 		if (*old_rate != new_rate)
 			*old_rate = new_rate;
-- 
2.31.1