drm: make frame duration time calculation more precise

It is a bit more precise to compute the total number of pixels first and
then divide, rather than multiplying the line pixel count by the
already-rounded line duration.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
index 71f820565637..38e79927b2d7 100644
--- a/drivers/gpu/drm/drm_irq.c
+++ b/drivers/gpu/drm/drm_irq.c
@@ -505,6 +505,7 @@ void drm_calc_timestamping_constants(struct drm_crtc *crtc)
 
         /* Valid dotclock? */
         if (dotclock > 0) {
+                int frame_size;
                 /* Convert scanline length in pixels and video dot clock to
                  * line duration, frame duration and pixel duration in
                  * nanoseconds:
@@ -512,7 +513,10 @@ void drm_calc_timestamping_constants(struct drm_crtc *crtc)
                 pixeldur_ns = (s64) div64_u64(1000000000, dotclock);
                 linedur_ns  = (s64) div64_u64(((u64) crtc->hwmode.crtc_htotal *
                                               1000000000), dotclock);
-                framedur_ns = (s64) crtc->hwmode.crtc_vtotal * linedur_ns;
+                frame_size = crtc->hwmode.crtc_htotal *
+                                crtc->hwmode.crtc_vtotal;
+                framedur_ns = (s64) div64_u64((u64) frame_size * 1000000000,
+                                              dotclock);
         } else
                 DRM_ERROR("crtc %d: Can't calculate constants, dotclock = 0!\n",
                           crtc->base.id);