linux-kernel - [PATCH 09/18] drm/sun4i: tcon: Adjust dotclock dividers range

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Date:   Thu, 13 Jul 2017 16:13:04 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Mark Brown <broonie@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:     dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh+dt@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        Boris Brezillon <boris.brezillon@...e-electrons.com>,
        Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Subject: [PATCH 09/18] drm/sun4i: tcon: Adjust dotclock dividers range

It seems like the dotclock dividers are a bit less strict range, and can
operate even with a smaller than 6 divider. Loose the boundaries a bit.

Signed-off-by: Maxime Ripard <maxime.ripard@...e-electrons.com>
---
 drivers/gpu/drm/sun4i/sun4i_dotclock.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/sun4i/sun4i_dotclock.c b/drivers/gpu/drm/sun4i/sun4i_dotclock.c
index d401156490f3..0b844c0dd102 100644
--- a/drivers/gpu/drm/sun4i/sun4i_dotclock.c
+++ b/drivers/gpu/drm/sun4i/sun4i_dotclock.c
@@ -77,7 +77,25 @@ static long sun4i_dclk_round_rate(struct clk_hw *hw, unsigned long rate,
 	u8 best_div = 1;
 	int i;
 
-	for (i = 6; i <= 127; i++) {
+	/*
+	 * There's something odd here.
+	 *
+	 * In the A13 user manual, this is stated to be >= 6 when
+	 * dclk1 and dclk2 are used (without any hint on how to use
+	 * them), and >= 4 when only dclk is used.
+	 *
+	 * In the A33 user manual, when only dclk is used, it is set
+	 * to be >= 6 in the former case, and >= 1 in the
+	 * latter. There's also some (obscure) explanations about the
+	 * dclk1 and dclk2 vs dclk that seems to be in the upper 4
+	 * bits. What those clocks are and what bit does what is not
+	 * really clear.
+	 *
+	 * On the A33 however, while something lower than 4 works, it
+	 * does have a few artifacts. Let's not use those values, and
+	 * see how it goes.
+	 */
+	for (i = 4; i <= 127; i++) {
 		unsigned long ideal = rate * i;
 		unsigned long rounded;
 
-- 
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