linux-kernel - [PATCH v1 1/7] clk: starfive: jh7100: Don't round divisor up twice

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Date:   Wed, 26 Jan 2022 18:39:47 +0100
From:   Emil Renner Berthing <kernel@...il.dk>
To:     linux-clk@...r.kernel.org, devicetree@...r.kernel.org
Cc:     Emil Renner Berthing <kernel@...il.dk>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        Michael Zhu <michael.zhu@...rfivetech.com>,
        Fu Wei <tekkamanninja@...il.com>, linux-kernel@...r.kernel.org
Subject: [PATCH v1 1/7] clk: starfive: jh7100: Don't round divisor up twice

The problem is best illustrated by an example. Suppose a consumer wants
a 4MHz clock rate from a divider with a 10MHz parent. It would then
call

  clk_round_rate(clk, 4000000)

which would call into our determine_rate() callback that correctly
rounds up and finds that a divisor of 3 gives the highest possible
frequency below the requested 4MHz and returns 10000000 / 3 = 3333333Hz.

However the consumer would then call

  clk_set_rate(clk, 3333333)

but since 3333333 doesn't divide 10000000 evenly our set_rate() callback
would again round the divisor up and set it to 4 which results in an
unnecessarily low rate of 2.5MHz.

Fix it by using DIV_ROUND_CLOSEST in the set_rate() callback.

Fixes: 4210be668a09 ("clk: starfive: Add JH7100 clock generator driver")
Signed-off-by: Emil Renner Berthing <kernel@...il.dk>
---
 drivers/clk/starfive/clk-starfive-jh7100.c | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/drivers/clk/starfive/clk-starfive-jh7100.c b/drivers/clk/starfive/clk-starfive-jh7100.c
index 25d31afa0f87..db6a4dc203af 100644
--- a/drivers/clk/starfive/clk-starfive-jh7100.c
+++ b/drivers/clk/starfive/clk-starfive-jh7100.c
@@ -399,22 +399,13 @@ static unsigned long jh7100_clk_recalc_rate(struct clk_hw *hw,
 	return div ? parent_rate / div : 0;
 }
 
-static unsigned long jh7100_clk_bestdiv(struct jh7100_clk *clk,
-					unsigned long rate, unsigned long parent)
-{
-	unsigned long max = clk->max_div;
-	unsigned long div = DIV_ROUND_UP(parent, rate);
-
-	return min(div, max);
-}
-
 static int jh7100_clk_determine_rate(struct clk_hw *hw,
 				     struct clk_rate_request *req)
 {
 	struct jh7100_clk *clk = jh7100_clk_from(hw);
 	unsigned long parent = req->best_parent_rate;
 	unsigned long rate = clamp(req->rate, req->min_rate, req->max_rate);
-	unsigned long div = jh7100_clk_bestdiv(clk, rate, parent);
+	unsigned long div = min_t(unsigned long, DIV_ROUND_UP(parent, rate), clk->max_div);
 	unsigned long result = parent / div;
 
 	/*
@@ -442,7 +433,8 @@ static int jh7100_clk_set_rate(struct clk_hw *hw,
 			       unsigned long parent_rate)
 {
 	struct jh7100_clk *clk = jh7100_clk_from(hw);
-	unsigned long div = jh7100_clk_bestdiv(clk, rate, parent_rate);
+	unsigned long div = clamp(DIV_ROUND_CLOSEST(parent_rate, rate),
+				  1UL, (unsigned long)clk->max_div);
 
 	jh7100_clk_reg_rmw(clk, JH7100_CLK_DIV_MASK, div);
 	return 0;
-- 
2.34.1