linux-kernel - [PATCH v3 01/13] clk: mmp: frac: Do not lose last 4 digits of precision

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Date:   Wed, 20 May 2020 00:41:39 +0200
From:   Lubomir Rintel <lkundrak@...sk>
To:     Stephen Boyd <sboyd@...nel.org>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh+dt@...nel.org>, linux-clk@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Lubomir Rintel <lkundrak@...sk>
Subject: [PATCH v3 01/13] clk: mmp: frac: Do not lose last 4 digits of precision

While calculating the output rate of a fractional divider clock, the
value is divided and multipled by 10000, discarding the least
significant digits -- presumably to fit the intermediate value within 32
bits.

The precision we're losing is, however, not insignificant for things like
I2S clock. Maybe also elsewhere, now that since commit ea56ad60260e ("clk:
mmp2: Stop pretending PLL outputs are constant") the parent rates are more
precise and no longer rounded to 10000s.

Signed-off-by: Lubomir Rintel <lkundrak@...sk>
---
 drivers/clk/mmp/clk-frac.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/drivers/clk/mmp/clk-frac.c b/drivers/clk/mmp/clk-frac.c
index fabc09aca6c4..ed9928f5bdc7 100644
--- a/drivers/clk/mmp/clk-frac.c
+++ b/drivers/clk/mmp/clk-frac.c
@@ -28,13 +28,15 @@ static long clk_factor_round_rate(struct clk_hw *hw, unsigned long drate,
 		unsigned long *prate)
 {
 	struct mmp_clk_factor *factor = to_clk_factor(hw);
-	unsigned long rate = 0, prev_rate;
+	u64 rate = 0, prev_rate;
 	int i;
 
 	for (i = 0; i < factor->ftbl_cnt; i++) {
 		prev_rate = rate;
-		rate = (((*prate / 10000) * factor->ftbl[i].den) /
-			(factor->ftbl[i].num * factor->masks->factor)) * 10000;
+		rate = *prate;
+		rate *= factor->ftbl[i].den;
+		do_div(rate, factor->ftbl[i].num * factor->masks->factor);
+
 		if (rate > drate)
 			break;
 	}
@@ -54,6 +56,7 @@ static unsigned long clk_factor_recalc_rate(struct clk_hw *hw,
 	struct mmp_clk_factor *factor = to_clk_factor(hw);
 	struct mmp_clk_factor_masks *masks = factor->masks;
 	unsigned int val, num, den;
+	u64 rate;
 
 	val = readl_relaxed(factor->base);
 
@@ -66,8 +69,11 @@ static unsigned long clk_factor_recalc_rate(struct clk_hw *hw,
 	if (!den)
 		return 0;
 
-	return (((parent_rate / 10000)  * den) /
-			(num * factor->masks->factor)) * 10000;
+	rate = parent_rate;
+	rate *= den;
+	do_div(rate, num * factor->masks->factor);
+
+	return rate;
 }
 
 /* Configures new clock rate*/
@@ -78,12 +84,14 @@ static int clk_factor_set_rate(struct clk_hw *hw, unsigned long drate,
 	struct mmp_clk_factor_masks *masks = factor->masks;
 	int i;
 	unsigned long val;
-	unsigned long rate = 0;
 	unsigned long flags = 0;
+	u64 rate = 0;
 
 	for (i = 0; i < factor->ftbl_cnt; i++) {
-		rate = (((prate / 10000) * factor->ftbl[i].den) /
-			(factor->ftbl[i].num * factor->masks->factor)) * 10000;
+		rate = prate;
+		rate *= factor->ftbl[i].den;
+		do_div(rate, factor->ftbl[i].num * factor->masks->factor);
+
 		if (rate > drate)
 			break;
 	}
-- 
2.26.2