drm/sun4i: dsi: Prevent underflow when computing packet sizes

Currently, the packet overhead is subtracted using unsigned arithmetic.
With a short sync pulse, this could underflow and wrap around to near
the maximal u16 value. Fix this by using signed subtraction. The call to
max() will correctly handle any negative numbers that are produced.

Apply the same fix to the other timings, even though those subtractions
are less likely to underflow.

Fixes: 133add5b ("drm/sun4i: Add Allwinner A31 MIPI-DSI controller support")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220812031623.34057-1-samuel@sholland.org

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c

@@ -531,7 +531,7 @@ static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
 				    struct drm_display_mode *mode)
 {
 	struct mipi_dsi_device *device = dsi->device;
-	unsigned int Bpp = mipi_dsi_pixel_format_to_bpp(device->format) / 8;
+	int Bpp = mipi_dsi_pixel_format_to_bpp(device->format) / 8;
 	u16 hbp = 0, hfp = 0, hsa = 0, hblk = 0, vblk = 0;
 	u32 basic_ctl = 0;
 	size_t bytes;
@@ -555,7 +555,7 @@ static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
 		 * (4 bytes). Its minimal size is therefore 10 bytes
 		 */
 #define HSA_PACKET_OVERHEAD	10
-		hsa = max((unsigned int)HSA_PACKET_OVERHEAD,
+		hsa = max(HSA_PACKET_OVERHEAD,
 			  (mode->hsync_end - mode->hsync_start) * Bpp - HSA_PACKET_OVERHEAD);
 
 		/*
@@ -564,7 +564,7 @@ static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
 		 * therefore 6 bytes
 		 */
 #define HBP_PACKET_OVERHEAD	6
-		hbp = max((unsigned int)HBP_PACKET_OVERHEAD,
+		hbp = max(HBP_PACKET_OVERHEAD,
 			  (mode->htotal - mode->hsync_end) * Bpp - HBP_PACKET_OVERHEAD);
 
 		/*
@@ -574,7 +574,7 @@ static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
 		 * 16 bytes
 		 */
 #define HFP_PACKET_OVERHEAD	16
-		hfp = max((unsigned int)HFP_PACKET_OVERHEAD,
+		hfp = max(HFP_PACKET_OVERHEAD,
 			  (mode->hsync_start - mode->hdisplay) * Bpp - HFP_PACKET_OVERHEAD);
 
 		/*
@@ -583,7 +583,7 @@ static void sun6i_dsi_setup_timings(struct sun6i_dsi *dsi,
 		 * bytes). Its minimal size is therefore 10 bytes.
 		 */
 #define HBLK_PACKET_OVERHEAD	10
-		hblk = max((unsigned int)HBLK_PACKET_OVERHEAD,
+		hblk = max(HBLK_PACKET_OVERHEAD,
 			   (mode->htotal - (mode->hsync_end - mode->hsync_start)) * Bpp -
 			   HBLK_PACKET_OVERHEAD);