kernel/bcm2835-clk-audio-jitter-issues.patch

From patchwork Thu Jun  1 14:14:16 2017
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Subject: [v4,1/2] clk: bcm2835: Limit PCM clock to OSC and PLLD_PER
From: Phil Elwell <phil@raspberrypi.org>
X-Patchwork-Id: 9759641
Message-Id: <8cc0ba82-d33e-127b-7b86-ac595ef416d1@raspberrypi.org>
To: Michael Turquette <mturquette@baylibre.com>,
 Stephen Boyd <sboyd@codeaurora.org>, Eric Anholt <eric@anholt.net>,
 Stefan Wahren <stefan.wahren@i2se.com>,
 Florian Fainelli <f.fainelli@gmail.com>,
 linux-clk@vger.kernel.org, linux-rpi-kernel@lists.infradead.org,
 linux-kernel@vger.kernel.org
Date: Thu, 1 Jun 2017 15:14:16 +0100

Restrict clock sources for the PCM peripheral to the oscillator and
PLLD_PER because other source may have varying rates or be switched off.
Prevent other sources from being selected by replacing their names in
the list of potential parents with dummy entries (entry index is
significant).

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
---
 drivers/clk/bcm/clk-bcm2835.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
index 0258538..49867d2 100644
--- a/drivers/clk/bcm/clk-bcm2835.c
+++ b/drivers/clk/bcm/clk-bcm2835.c
@@ -1516,6 +1516,31 @@ struct bcm2835_clk_desc {
 	.parents = bcm2835_clock_per_parents,				\
 	__VA_ARGS__)
 
+/*
+ * Restrict clock sources for the PCM peripheral to the oscillator and
+ * PLLD_PER because other source may have varying rates or be switched
+ * off.
+ *
+ * Prevent other sources from being selected by replacing their names in
+ * the list of potential parents with dummy entries (entry index is
+ * significant).
+ */
+static const char *const bcm2835_pcm_per_parents[] = {
+	"-",
+	"xosc",
+	"-",
+	"-",
+	"-",
+	"-",
+	"plld_per",
+	"-",
+};
+
+#define REGISTER_PCM_CLK(...)	REGISTER_CLK(				\
+	.num_mux_parents = ARRAY_SIZE(bcm2835_pcm_per_parents),		\
+	.parents = bcm2835_pcm_per_parents,				\
+	__VA_ARGS__)
+
 /* main vpu parent mux */
 static const char *const bcm2835_clock_vpu_parents[] = {
 	"gnd",
@@ -1993,7 +2018,7 @@ struct bcm2835_clk_desc {
 		.int_bits = 4,
 		.frac_bits = 8,
 		.tcnt_mux = 22),
-	[BCM2835_CLOCK_PCM]	= REGISTER_PER_CLK(
+	[BCM2835_CLOCK_PCM]	= REGISTER_PCM_CLK(
 		.name = "pcm",
 		.ctl_reg = CM_PCMCTL,
 		.div_reg = CM_PCMDIV,
From patchwork Thu Jun  1 14:14:22 2017
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Subject: [v4,2/2] clk: bcm2835: Minimise clock jitter for PCM clock
From: Phil Elwell <phil@raspberrypi.org>
X-Patchwork-Id: 9759643
Message-Id: <9989244b-ca4d-9081-95d9-b24f51099222@raspberrypi.org>
To: Michael Turquette <mturquette@baylibre.com>,
 Stephen Boyd <sboyd@codeaurora.org>, Eric Anholt <eric@anholt.net>,
 Stefan Wahren <stefan.wahren@i2se.com>,
 Florian Fainelli <f.fainelli@gmail.com>,
 linux-clk@vger.kernel.org, linux-rpi-kernel@lists.infradead.org,
 linux-kernel@vger.kernel.org
Date: Thu, 1 Jun 2017 15:14:22 +0100

Fractional clock dividers generate accurate average frequencies but
with jitter, particularly when the integer divisor is small.

Introduce a new metric of clock accuracy to penalise clocks with a good
average but worse jitter compared to clocks with an average which is no
better but with lower jitter. The metric is the ideal rate minus the
worse deviation from that ideal using the nearest integer divisors.

Use this metric for parent selection for clocks requiring low jitter
(currently just PCM).

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
---
 drivers/clk/bcm/clk-bcm2835.c | 34 +++++++++++++++++++++++++++++-----
 1 file changed, 29 insertions(+), 5 deletions(-)

diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
index 49867d2..0bc56a0 100644
--- a/drivers/clk/bcm/clk-bcm2835.c
+++ b/drivers/clk/bcm/clk-bcm2835.c
@@ -530,6 +530,7 @@ struct bcm2835_clock_data {
 
 	bool is_vpu_clock;
 	bool is_mash_clock;
+	bool low_jitter;
 
 	u32 tcnt_mux;
 };
@@ -1124,7 +1125,8 @@ static unsigned long bcm2835_clock_choose_div_and_prate(struct clk_hw *hw,
 							int parent_idx,
 							unsigned long rate,
 							u32 *div,
-							unsigned long *prate)
+							unsigned long *prate,
+							unsigned long *avgrate)
 {
 	struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
 	struct bcm2835_cprman *cprman = clock->cprman;
@@ -1139,8 +1141,25 @@ static unsigned long bcm2835_clock_choose_div_and_prate(struct clk_hw *hw,
 		*prate = clk_hw_get_rate(parent);
 		*div = bcm2835_clock_choose_div(hw, rate, *prate, true);
 
-		return bcm2835_clock_rate_from_divisor(clock, *prate,
-						       *div);
+		*avgrate = bcm2835_clock_rate_from_divisor(clock, *prate, *div);
+
+		if (data->low_jitter && (*div & CM_DIV_FRAC_MASK)) {
+			unsigned long high, low;
+			u32 int_div = *div & ~CM_DIV_FRAC_MASK;
+
+			high = bcm2835_clock_rate_from_divisor(clock, *prate,
+							       int_div);
+			int_div += CM_DIV_FRAC_MASK + 1;
+			low = bcm2835_clock_rate_from_divisor(clock, *prate,
+							      int_div);
+
+			/*
+			 * Return a value which is the maximum deviation
+			 * below the ideal rate, for use as a metric.
+			 */
+			return *avgrate - max(*avgrate - low, high - *avgrate);
+		}
+		return *avgrate;
 	}
 
 	if (data->frac_bits)
@@ -1167,6 +1186,7 @@ static unsigned long bcm2835_clock_choose_div_and_prate(struct clk_hw *hw,
 
 	*div = curdiv << CM_DIV_FRAC_BITS;
 	*prate = curdiv * best_rate;
+	*avgrate = best_rate;
 
 	return best_rate;
 }
@@ -1178,6 +1198,7 @@ static int bcm2835_clock_determine_rate(struct clk_hw *hw,
 	bool current_parent_is_pllc;
 	unsigned long rate, best_rate = 0;
 	unsigned long prate, best_prate = 0;
+	unsigned long avgrate, best_avgrate = 0;
 	size_t i;
 	u32 div;
 
@@ -1202,11 +1223,13 @@ static int bcm2835_clock_determine_rate(struct clk_hw *hw,
 			continue;
 
 		rate = bcm2835_clock_choose_div_and_prate(hw, i, req->rate,
-							  &div, &prate);
+							  &div, &prate,
+							  &avgrate);
 		if (rate > best_rate && rate <= req->rate) {
 			best_parent = parent;
 			best_prate = prate;
 			best_rate = rate;
+			best_avgrate = avgrate;
 		}
 	}
 
@@ -1216,7 +1239,7 @@ static int bcm2835_clock_determine_rate(struct clk_hw *hw,
 	req->best_parent_hw = best_parent;
 	req->best_parent_rate = best_prate;
 
-	req->rate = best_rate;
+	req->rate = best_avgrate;
 
 	return 0;
 }
@@ -2025,6 +2048,7 @@ struct bcm2835_clk_desc {
 		.int_bits = 12,
 		.frac_bits = 12,
 		.is_mash_clock = true,
+		.low_jitter = true,
 		.tcnt_mux = 23),
 	[BCM2835_CLOCK_PWM]	= REGISTER_PER_CLK(
 		.name = "pwm",