linux-kernel - [PATCH 4.17 056/336] clk-si544: Properly round requested frequency to nearest match

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Date:   Wed,  1 Aug 2018 18:46:31 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Mike Looijmans <mike.looijmans@...ic.nl>,
        Stephen Boyd <sboyd@...nel.org>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: [PATCH 4.17 056/336] clk-si544: Properly round requested frequency to nearest match

4.17-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mike Looijmans <mike.looijmans@...ic.nl>

[ Upstream commit 4d3f36c5e9ca0f947eed71660239c529c501141a ]

The si544 driver had a rounding problem that using the result of clk_round_rate
may set the clock to yet another rate, for example:
clk_round_rate(195000000) = 194999999
clk_round_rate(194999999) = 194999998

Clients would expect that after clk_set_rate(clk, freq2=clk_round_rate(clk, freq)) the
chip will be running at exactly freq2.

The problem was in the calculation of the feedback divider, it was always rounded
down instead of to the nearest possible VCO value.

After this change, the following holds true for any supported frequency:
actual_freq = clk_round_rate(clk, freq);
clk_set_rate(clk, actual_freq);
clk_round_rate(clk, actual_freq) == actual_freq && clk_get_rate(clk) == actual_freq

Signed-off-by: Mike Looijmans <mike.looijmans@...ic.nl>
Fixes: 953cc3e81170 ("clk: Add driver for the si544 clock generator chip")
Signed-off-by: Stephen Boyd <sboyd@...nel.org>
Signed-off-by: Sasha Levin <alexander.levin@...rosoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/clk/clk-si544.c |    1 +
 1 file changed, 1 insertion(+)

--- a/drivers/clk/clk-si544.c
+++ b/drivers/clk/clk-si544.c
@@ -207,6 +207,7 @@ static int si544_calc_muldiv(struct clk_
 
 	/* And the fractional bits using the remainder */
 	vco = (u64)tmp << 32;
+	vco += FXO / 2; /* Round to nearest multiple */
 	do_div(vco, FXO);
 	settings->fb_div_frac = vco;