drm/tilcdc: Precalculate total frametime in tilcdc_crtc_set_mode()

We need the total frame refresh time to check if we are too close to
vertical sync when updating the two framebuffer DMA registers and risk
a collision. This new method is more accurate that the previous that
based on mode's vrefresh value, which itself is inaccurate or may not
even be initialized.

Reported-by: Kevin Hao <kexin.hao@windriver.com>
Fixes: 11abbc9 ("drm/tilcdc: Set framebuffer DMA address to HW only if CRTC is enabled")
Cc: <stable@vger.kernel.org> # v4.11+
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/gpu/drm/tilcdc/tilcdc_crtc.c

@@ -24,6 +24,7 @@
 #include <linux/completion.h>
 #include <linux/dma-mapping.h>
 #include <linux/of_graph.h>
+#include <linux/math64.h>
 
 #include "tilcdc_drv.h"
 #include "tilcdc_regs.h"
@@ -48,6 +49,7 @@ struct tilcdc_crtc {
 	unsigned int lcd_fck_rate;
 
 	ktime_t last_vblank;
+	unsigned int hvtotal_us;
 
 	struct drm_framebuffer *curr_fb;
 	struct drm_framebuffer *next_fb;
@@ -292,6 +294,12 @@ static void tilcdc_crtc_set_clk(struct drm_crtc *crtc)
 				LCDC_V2_CORE_CLK_EN);
 }
 
+uint tilcdc_mode_hvtotal(const struct drm_display_mode *mode)
+{
+	return (uint) div_u64(1000llu * mode->htotal * mode->vtotal,
+			      mode->clock);
+}
+
 static void tilcdc_crtc_set_mode(struct drm_crtc *crtc)
 {
 	struct tilcdc_crtc *tilcdc_crtc = to_tilcdc_crtc(crtc);
@@ -459,6 +467,9 @@ static void tilcdc_crtc_set_mode(struct drm_crtc *crtc)
 	drm_framebuffer_get(fb);
 
 	crtc->hwmode = crtc->state->adjusted_mode;
+
+	tilcdc_crtc->hvtotal_us =
+		tilcdc_mode_hvtotal(&crtc->hwmode);
 }
 
 static void tilcdc_crtc_enable(struct drm_crtc *crtc)
@@ -642,7 +653,7 @@ int tilcdc_crtc_update_fb(struct drm_crtc *crtc,
 		spin_lock_irqsave(&tilcdc_crtc->irq_lock, flags);
 
 		next_vblank = ktime_add_us(tilcdc_crtc->last_vblank,
-					   1000000 / crtc->hwmode.vrefresh);
+					   tilcdc_crtc->hvtotal_us);
 		tdiff = ktime_to_us(ktime_sub(next_vblank, ktime_get()));
 
 		if (tdiff < TILCDC_VBLANK_SAFETY_THRESHOLD_US)