drm/mediatek: dsi: Modify horizontal front/back porch byte formula

In the patch to be fixed, horizontal_backporch_byte become too large
for some panel, so roll back that patch. For small hfp or hbp panel,
using vm->hfront_porch + vm->hback_porch to calculate
horizontal_backporch_byte would make it negtive, so
use horizontal_backporch_byte itself to make it positive.

Fixes: 35bf948f ("drm/mediatek: dsi: Fix scrolling of panel with small hfp or hbp")
Signed-off-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Tested-by: Bilal Wasim <bilal.wasim@imgtec.com>

drivers/gpu/drm/mediatek/mtk_dsi.c

@@ -444,7 +444,10 @@ static void mtk_dsi_config_vdo_timing(struct mtk_dsi *dsi)
 	u32 horizontal_sync_active_byte;
 	u32 horizontal_backporch_byte;
 	u32 horizontal_frontporch_byte;
+	u32 horizontal_front_back_byte;
+	u32 data_phy_cycles_byte;
 	u32 dsi_tmp_buf_bpp, data_phy_cycles;
+	u32 delta;
 	struct mtk_phy_timing *timing = &dsi->phy_timing;
 
 	struct videomode *vm = &dsi->vm;
@@ -466,50 +469,30 @@ static void mtk_dsi_config_vdo_timing(struct mtk_dsi *dsi)
 	horizontal_sync_active_byte = (vm->hsync_len * dsi_tmp_buf_bpp - 10);
 
 	if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO_SYNC_PULSE)
-		horizontal_backporch_byte = vm->hback_porch * dsi_tmp_buf_bpp;
+		horizontal_backporch_byte = vm->hback_porch * dsi_tmp_buf_bpp - 10;
 	else
 		horizontal_backporch_byte = (vm->hback_porch + vm->hsync_len) *
-					    dsi_tmp_buf_bpp;
+					    dsi_tmp_buf_bpp - 10;
 
 	data_phy_cycles = timing->lpx + timing->da_hs_prepare +
-			  timing->da_hs_zero + timing->da_hs_exit;
-
-	if (dsi->mode_flags & MIPI_DSI_MODE_VIDEO_BURST) {
-		if ((vm->hfront_porch + vm->hback_porch) * dsi_tmp_buf_bpp >
-		    data_phy_cycles * dsi->lanes + 18) {
-			horizontal_frontporch_byte =
-				vm->hfront_porch * dsi_tmp_buf_bpp -
-				(data_phy_cycles * dsi->lanes + 18) *
-				vm->hfront_porch /
-				(vm->hfront_porch + vm->hback_porch);
-
-			horizontal_backporch_byte =
-				horizontal_backporch_byte -
-				(data_phy_cycles * dsi->lanes + 18) *
-				vm->hback_porch /
-				(vm->hfront_porch + vm->hback_porch);
-		} else {
-			DRM_WARN("HFP less than d-phy, FPS will under 60Hz\n");
-			horizontal_frontporch_byte = vm->hfront_porch *
-						     dsi_tmp_buf_bpp;
-		}
+			  timing->da_hs_zero + timing->da_hs_exit + 3;
+
+	delta = dsi->mode_flags & MIPI_DSI_MODE_VIDEO_BURST ? 18 : 12;
+
+	horizontal_frontporch_byte = vm->hfront_porch * dsi_tmp_buf_bpp;
+	horizontal_front_back_byte = horizontal_frontporch_byte + horizontal_backporch_byte;
+	data_phy_cycles_byte = data_phy_cycles * dsi->lanes + delta;
+
+	if (horizontal_front_back_byte > data_phy_cycles_byte) {
+		horizontal_frontporch_byte -= data_phy_cycles_byte *
+					      horizontal_frontporch_byte /
+					      horizontal_front_back_byte;
+
+		horizontal_backporch_byte -= data_phy_cycles_byte *
+					     horizontal_backporch_byte /
+					     horizontal_front_back_byte;
 	} else {
-		if ((vm->hfront_porch + vm->hback_porch) * dsi_tmp_buf_bpp >
-		    data_phy_cycles * dsi->lanes + 12) {
-			horizontal_frontporch_byte =
-				vm->hfront_porch * dsi_tmp_buf_bpp -
-				(data_phy_cycles * dsi->lanes + 12) *
-				vm->hfront_porch /
-				(vm->hfront_porch + vm->hback_porch);
-			horizontal_backporch_byte = horizontal_backporch_byte -
-				(data_phy_cycles * dsi->lanes + 12) *
-				vm->hback_porch /
-				(vm->hfront_porch + vm->hback_porch);
-		} else {
-			DRM_WARN("HFP less than d-phy, FPS will under 60Hz\n");
-			horizontal_frontporch_byte = vm->hfront_porch *
-						     dsi_tmp_buf_bpp;
-		}
+		DRM_WARN("HFP + HBP less than d-phy, FPS will under 60Hz\n");
 	}
 
 	writel(horizontal_sync_active_byte, dsi->regs + DSI_HSA_WC);