[PATCH 31/72] gpu: ipu-di: Set rate of DI pre clock

[Steve Longerbeam <slongerbeam@gmail.com>]

This patch sets the rate of the DI pre clock to support a much wider
range of pixel clock frequencies.

It does this by calculating two values for the pre-clk rate: a
rate that is a whole integer multiple of the pixel clock, and a rate
that is a half-integer multiple. It then programs whichever rate
comes closest to generating the desired pixel clock, and uses the
corresponding integer or half-integer divider.

The reason only whole or half integer DI dividers are used is because
of a chip bug in the fractional part of DI Base Sync Clock Gen 0 register
(see discussion in FSL community site at
https://community.freescale.com/thread/308577).

Quoting from that thread:

"Pixel IPU clock for Display has high jitter and does not have 50% duty cycle.
This is expected behavior, when the clock divider is set to a value that is not
an integer. When the divider is set to a non-integer value, the average
frequency of the clock will be correct, but the clock will jitter."

According to our experimentation, only 0x8 for the fractional part seems
to generate clock waveforms that are tolerated by most displays (this
hasn't been proved officially).

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
---
 drivers/gpu/ipu-v3/ipu-di.c |  145 +++++++++++++++++++++++++++++++++----------
 1 file changed, 111 insertions(+), 34 deletions(-)

diff --git a/drivers/gpu/ipu-v3/ipu-di.c b/drivers/gpu/ipu-v3/ipu-di.c
index b306f07..8da9c9f 100644
--- a/drivers/gpu/ipu-v3/ipu-di.c
+++ b/drivers/gpu/ipu-v3/ipu-di.c
@@ -398,12 +398,105 @@ static void ipu_di_sync_config_noninterlaced(struct ipu_di *di,
 		ipu_di_sync_config(di, cfg_vga, 0, ARRAY_SIZE(cfg_vga));
 }
 
-static void ipu_di_config_clock(struct ipu_di *di,
-	const struct ipu_di_signal_cfg *sig)
+
+/*
+ * We need to use the DI divider. We should really have a flag here
+ * indicating whether the bridge can cope with a fractional divider
+ * or not.
+ *
+ * For now, assume the chip bug exists in the fractional divider
+ * which causes bad DI pixel clock waveforms for all values of the
+ * fractional part other than 0 (no fraction) or 0x8 (0.5). So this
+ * function returns a divider with only 0x0 or 0x8 in the fractional
+ * part.
+ *
+ * This function reprograms the clock rate of the parent-parent of
+ * the DI clock in order to get as close as possible to the requested
+ * pixel clock. It calculates two values for the parent-parent rate: a
+ * rate that is a whole integer multiple of the pixel clock, and a rate
+ * that is a half-integer multiple. It then programs whichever rate
+ * comes closest to generating the desired rate, and returns the
+ * corresponding integer or half-integer divider (times 16).
+ */
+static int set_di_pre_clk_rate(struct ipu_di *di,
+			       const struct ipu_di_signal_cfg *sig)
+{
+	struct clk *di_pre_clk;
+	unsigned long pre_clk_N_round, pre_clk_N_0_5_round;
+	unsigned long pre_clk_N, pre_clk_N_0_5;
+	unsigned long pixclk_N, pixclk_N_0_5;
+	unsigned div_N, div_N_0_5, error_N, error_N_0_5;
+	int ret;
+
+	di_pre_clk = clk_get_parent(clk_get_parent(di->clk_di));
+	if (IS_ERR(di_pre_clk)) {
+		dev_err(di->ipu->dev, "failed to get di_pre clock\n");
+		return PTR_ERR(di_pre_clk);
+	}
+
+	/*
+	 * calc pre_clk_N and pre_clk_N_0_5, which are an integer multiple
+	 * and half-integer multiple of sig->pixelclock, respectively.
+	 */
+	pre_clk_N = sig->pixelclock;
+	pre_clk_N_round = clk_round_rate(di_pre_clk, pre_clk_N);
+	pre_clk_N *= (pre_clk_N_round / pre_clk_N);
+	pre_clk_N_0_5 = pre_clk_N + sig->pixelclock / 2;
+
+	if (pre_clk_N < pre_clk_N_round)
+		pre_clk_N += sig->pixelclock;
+	if (pre_clk_N_0_5 < pre_clk_N_round)
+		pre_clk_N_0_5 += sig->pixelclock;
+
+	/*
+	 * now get the rounded pre_clk_N and pre_clk_N_0_5, i.e. what the
+	 * pre-clk can actually generate.
+	 */
+	pre_clk_N_round = clk_round_rate(di_pre_clk, pre_clk_N);
+	pre_clk_N_0_5_round = clk_round_rate(di_pre_clk, pre_clk_N_0_5);
+
+	/*
+	 * finally we can determine whether the integer multiple or
+	 * half-integer multiple pre-clk comes closest to generating
+	 * the desired pixel clock. Set pre-clk rate to whichever comes
+	 * closest.
+	 */
+	div_N = (pre_clk_N_round << 4) / sig->pixelclock;
+	div_N = (div_N + 0x8) & ~0xf; /* round to nearest int */
+	pixclk_N = (pre_clk_N_round << 4) / div_N;
+
+	div_N_0_5 = (pre_clk_N_0_5_round << 4) / sig->pixelclock;
+	div_N_0_5 = (div_N_0_5 + 0x4) & ~0x7; /* round to nearest half-int */
+	pixclk_N_0_5 = (pre_clk_N_0_5_round << 4) / div_N_0_5;
+
+	error_N = pixclk_N / (sig->pixelclock / 1000);
+	error_N_0_5 = pixclk_N_0_5 / (sig->pixelclock / 1000);
+
+	if (abs(error_N - 1000) < abs(error_N_0_5 - 1000)) {
+		ret = div_N;
+		clk_set_rate(di_pre_clk, pre_clk_N_round);
+	} else {
+		ret = div_N_0_5;
+		clk_set_rate(di_pre_clk, pre_clk_N_0_5_round);
+	}
+
+	dev_dbg(di->ipu->dev,
+		"di%d: pixclk want %lu, can do %lu / %lu, div 0x%02x / 0x%02x, error %d.%u%% / %d.%u%%\n",
+		di->id, sig->pixelclock, pixclk_N, pixclk_N_0_5,
+		div_N, div_N_0_5,
+		(signed)(error_N - 1000) / 10, error_N % 10,
+		(signed)(error_N_0_5 - 1000) / 10, error_N_0_5 % 10);
+
+	return ret;
+}
+
+static int ipu_di_config_clock(struct ipu_di *di,
+			       const struct ipu_di_signal_cfg *sig)
 {
 	struct clk *clk;
 	unsigned clkgen0;
 	uint32_t val;
+	int ret;
 
 	if (sig->clkflags & IPU_DI_CLKMODE_EXT) {
 		/*
@@ -423,24 +516,10 @@ static void ipu_di_config_clock(struct ipu_di *di,
 			 */
 			clkgen0 = 1 << 4;
 		} else {
-			/*
-			 * We can use the divider.  We should really have
-			 * a flag here indicating whether the bridge can
-			 * cope with a fractional divider or not.  For the
-			 * time being, let's go for simplicitly and
-			 * reliability.
-			 */
-			unsigned long in_rate;
-			unsigned div;
-
-			clk_set_rate(clk, sig->pixelclock);
-
-			in_rate = clk_get_rate(clk);
-			div = (in_rate + sig->pixelclock / 2) / sig->pixelclock;
-			if (div == 0)
-				div = 1;
-
-			clkgen0 = div << 4;
+			ret = set_di_pre_clk_rate(di, sig);
+			if (ret < 0)
+				return ret;
+			clkgen0 = ret;
 		}
 	} else {
 		/*
@@ -470,19 +549,12 @@ static void ipu_di_config_clock(struct ipu_di *di,
 
 			clkgen0 = div << 4;
 		} else {
-			unsigned long in_rate;
-			unsigned div;
-
 			clk = di->clk_di;
 
-			clk_set_rate(clk, sig->pixelclock);
-
-			in_rate = clk_get_rate(clk);
-			div = (in_rate + sig->pixelclock / 2) / sig->pixelclock;
-			if (div == 0)
-				div = 1;
-
-			clkgen0 = div << 4;
+			ret = set_di_pre_clk_rate(di, sig);
+			if (ret < 0)
+				return ret;
+			clkgen0 = ret;
 		}
 	}
 
@@ -513,6 +585,8 @@ static void ipu_di_config_clock(struct ipu_di *di,
 		clk_get_rate(di->clk_di),
 		clk == di->clk_di ? "DI" : "IPU",
 		clk_get_rate(di->clk_di_pixel) / (clkgen0 >> 4));
+
+	return 0;
 }
 
 /*
@@ -546,6 +620,7 @@ int ipu_di_init_sync_panel(struct ipu_di *di, struct ipu_di_signal_cfg *sig)
 	u32 reg;
 	u32 di_gen, vsync_cnt;
 	u32 div;
+	int ret = 0;
 
 	dev_dbg(di->ipu->dev, "di%d: panel size = %d x %d\n",
 		di->id, sig->width, sig->height);
@@ -564,7 +639,9 @@ int ipu_di_init_sync_panel(struct ipu_di *di, struct ipu_di_signal_cfg *sig)
 
 	mutex_lock(&di_mutex);
 
-	ipu_di_config_clock(di, sig);
+	ret = ipu_di_config_clock(di, sig);
+	if (ret)
+		goto unlock;
 
 	div = ipu_di_read(di, DI_BS_CLKGEN0) & 0xfff;
 	div = div / 16;		/* Now divider is integer portion */
@@ -641,9 +718,9 @@ int ipu_di_init_sync_panel(struct ipu_di *di, struct ipu_di_signal_cfg *sig)
 
 	ipu_di_write(di, reg, DI_POL);
 
+unlock:
 	mutex_unlock(&di_mutex);
-
-	return 0;
+	return ret;
 }
 EXPORT_SYMBOL_GPL(ipu_di_init_sync_panel);
 
-- 
1.7.9.5

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