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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Dave Stevenson <dave.stevenson@raspberrypi.com>,
        Maxime Ripard <maxime@cerno.tech>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.19 078/287] drm/vc4: dsi: Correct DSI divider calculations
Date:   Tue, 23 Aug 2022 10:24:07 +0200
Message-Id: <20220823080102.877281360@linuxfoundation.org>
In-Reply-To: <20220823080100.268827165@linuxfoundation.org>
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From: Dave Stevenson <dave.stevenson@raspberrypi.com>

[ Upstream commit 3b45eee87da171caa28f61240ddb5c21170cda53 ]

The divider calculations tried to find the divider just faster than the
clock requested. However if it required a divider of 7 then the for loop
aborted without handling the "error" case, and could end up with a clock
lower than requested.

The integer divider from parent PLL to DSI clock is also capable of
going up to /255, not just /7 that the driver was trying.  This allows
for slower link frequencies on the DSI bus where the resolution permits.

Correct the loop so that we always have a clock greater than requested,
and covering the whole range of dividers.

Fixes: 86c1b9eff3f2 ("drm/vc4: Adjust modes in DSI to work around the integer PLL divider.")
Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://lore.kernel.org/r/20220613144800.326124-13-maxime@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/gpu/drm/vc4/vc4_dsi.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c
index 0c607eb33d7e..77003ce666a4 100644
--- a/drivers/gpu/drm/vc4/vc4_dsi.c
+++ b/drivers/gpu/drm/vc4/vc4_dsi.c
@@ -853,11 +853,9 @@ static bool vc4_dsi_encoder_mode_fixup(struct drm_encoder *encoder,
 	/* Find what divider gets us a faster clock than the requested
 	 * pixel clock.
 	 */
-	for (divider = 1; divider < 8; divider++) {
-		if (parent_rate / divider < pll_clock) {
-			divider--;
+	for (divider = 1; divider < 255; divider++) {
+		if (parent_rate / (divider + 1) < pll_clock)
 			break;
-		}
 	}
 
 	/* Now that we've picked a PLL divider, calculate back to its
-- 
2.35.1