[PATCH] drm/sun4i: dsi: Prevent underflow when computing packet sizes

[Samuel Holland]

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: 133add5b5ad4 ("drm/sun4i: Add Allwinner A31 MIPI-DSI controller support")
Signed-off-by: Samuel Holland <samuel at sholland.org>
---

 drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
index b4dfa166eccd..34234a144e87 100644
--- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
+++ b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
@@ -522,77 +522,77 @@ static void sun6i_dsi_setup_format(struct sun6i_dsi *dsi,
 		     SUN6I_DSI_PIXEL_PF1_CRC_INIT_LINE0(0xffff) |
 		     SUN6I_DSI_PIXEL_PF1_CRC_INIT_LINEN(0xffff));
 
 	regmap_write(dsi->regs, SUN6I_DSI_PIXEL_CTL0_REG,
 		     SUN6I_DSI_PIXEL_CTL0_PD_PLUG_DISABLE |
 		     SUN6I_DSI_PIXEL_CTL0_FORMAT(fmt));
 }
 
 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;
 	u8 *buffer;
 
 	/* Do all timing calculations up front to allocate buffer space */
 
 	if (device->mode_flags & MIPI_DSI_MODE_VIDEO_BURST) {
 		hblk = mode->hdisplay * Bpp;
 		basic_ctl = SUN6I_DSI_BASIC_CTL_VIDEO_BURST |
 			    SUN6I_DSI_BASIC_CTL_HSA_HSE_DIS |
 			    SUN6I_DSI_BASIC_CTL_HBP_DIS;
 
 		if (device->lanes == 4)
 			basic_ctl |= SUN6I_DSI_BASIC_CTL_TRAIL_FILL |
 				     SUN6I_DSI_BASIC_CTL_TRAIL_INV(0xc);
 	} else {
 		/*
 		 * A sync period is composed of a blanking packet (4
 		 * bytes + payload + 2 bytes) and a sync event packet
 		 * (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);
 
 		/*
 		 * The backporch is set using a blanking packet (4
 		 * bytes + payload + 2 bytes). Its minimal size is
 		 * 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);
 
 		/*
 		 * The frontporch is set using a sync event (4 bytes)
 		 * and two blanking packets (each one is 4 bytes +
 		 * payload + 2 bytes). Its minimal size is therefore
 		 * 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);
 
 		/*
 		 * The blanking is set using a sync event (4 bytes)
 		 * and a blanking packet (4 bytes + payload + 2
 		 * 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);
 
 		/*
 		 * And I'm not entirely sure what vblk is about. The driver in
 		 * Allwinner BSP is using a rather convoluted calculation
 		 * there only for 4 lanes. However, using 0 (the !4 lanes
 		 * case) even with a 4 lanes screen seems to work...
 		 */
 		vblk = 0;
 	}
 
-- 
2.35.1