drm/exynos: use 4WORD dma burst length for small fbs

In case of exynos, setting dma-burst to 16Word causes permanent
tearing for very small buffers, e.g. cursor buffer. Burst Mode
switching, which is based on overlay size is not recommended as
overlay size varies a lot towards the end of the screen. This
causes unstable DMA which results into tearing again.

Rendering small buffers with lower burst size doesn't
cause any noticable performance overhead. 128 pixel width is
selected based on mulitple experiments with exynos5 SoCs.
Signed-off-by: Rahul Sharma <Rahul.Sharma@samsung.com>
Signed-off-by: Prathyush K <prathyush.k@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_fimd.c

@@ -39,6 +39,7 @@
  */
 
 #define FIMD_DEFAULT_FRAMERATE 60
+#define MIN_FB_WIDTH_FOR_16WORD_BURST 128
 
 /* position control register for hardware window 0, 2 ~ 4.*/
 #define VIDOSD_A(win)		(VIDOSD_BASE + 0x00 + (win) * 16)
@@ -479,6 +480,19 @@ static void fimd_win_set_pixfmt(struct fimd_context *ctx, unsigned int win)
 
 	DRM_DEBUG_KMS("bpp = %d\n", win_data->bpp);
 
+	/*
+	 * In case of exynos, setting dma-burst to 16Word causes permanent
+	 * tearing for very small buffers, e.g. cursor buffer. Burst Mode
+	 * switching which is based on overlay size is not recommended as
+	 * overlay size varies alot towards the end of the screen and rapid
+	 * movement causes unstable DMA which results into iommu crash/tear.
+	 */
+
+	if (win_data->fb_width < MIN_FB_WIDTH_FOR_16WORD_BURST) {
+		val &= ~WINCONx_BURSTLEN_MASK;
+		val |= WINCONx_BURSTLEN_4WORD;
+	}
+
 	writel(val, ctx->regs + WINCON(win));
 }