linux-kernel - [PATCH] drm/rockchip: cdn-dp: fix sign extension on an int multiply for a u64 result

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Date:   Tue, 15 Sep 2020 17:20:49 +0100
From:   Colin King <colin.king@...onical.com>
To:     Sandy Huang <hjc@...k-chips.com>,
        Heiko Stübner <heiko@...ech.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Chris Zhong <zyw@...k-chips.com>,
        Guenter Roeck <groeck@...omium.org>,
        Sean Paul <seanpaul@...omium.org>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] drm/rockchip: cdn-dp: fix sign extension on an int multiply for a u64 result

From: Colin Ian King <colin.king@...onical.com>

The variable bit_per_pix is a u8 and is promoted in the multiplication
to an int type and then sign extended to a u64. If the result of the
int multiplication is greater than 0x7fffffff then the upper 32 bits will
be set to 1 as a result of the sign extension. Avoid this by casting
tu_size_reg to u64 to avoid sign extension and also a potential overflow.

Addresses-Coverity: ("Unintended sign extension")
Fixes: 1a0f7ed3abe2 ("drm/rockchip: cdn-dp: add cdn DP support for rk3399")
Signed-off-by: Colin Ian King <colin.king@...onical.com>
---
 drivers/gpu/drm/rockchip/cdn-dp-reg.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/rockchip/cdn-dp-reg.c b/drivers/gpu/drm/rockchip/cdn-dp-reg.c
index 9d2163ef4d6e..33fb4d05c506 100644
--- a/drivers/gpu/drm/rockchip/cdn-dp-reg.c
+++ b/drivers/gpu/drm/rockchip/cdn-dp-reg.c
@@ -658,7 +658,7 @@ int cdn_dp_config_video(struct cdn_dp_device *dp)
 	 */
 	do {
 		tu_size_reg += 2;
-		symbol = tu_size_reg * mode->clock * bit_per_pix;
+		symbol = (u64)tu_size_reg * mode->clock * bit_per_pix;
 		do_div(symbol, dp->max_lanes * link_rate * 8);
 		rem = do_div(symbol, 1000);
 		if (tu_size_reg > 64) {
-- 
2.27.0