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

[ Upstream commit 82a1356a933d8443139f8886f11b63c974a09a67 ]

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@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
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c b/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
index 4f5efcace68ea515c1114224c31f65c86f742f65..51edb4244af7c2cb5d71c393d1a881b5e0707e69 100644 (file)
--- a/drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c
+++ b/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);