[PATCH v2] clk: bcm2835: Round UART input clock up

[Ivan T. Ivanov]

The UART clock is initialised to be as close to the requested
frequency as possible without exceeding it. Now that there is a
clock manager that returns the actual frequencies, an expected
48MHz clock is reported as 47999625. If the requested baudrate
== requested clock/16, there is no headroom and the slight
reduction in actual clock rate results in failure.

If increasing a clock by less than 0.1% changes it from ..999..
to ..000.., round it up.

This is reworked version of a downstream fix:
https://github.com/raspberrypi/linux/commit/ab3f1b39537f6d3825b8873006fbe2fc5ff057b7

Cc: Phil Elwell <phil at raspberrypi.org>
Signed-off-by: Ivan T. Ivanov <iivanov at suse.de>
---
Changes since v1
Make bcm2835_clock_round() static to fix following warning
when compiling for riscv:
drivers/clk/bcm/clk-bcm2835.c:997:15: warning: no previous prototype for 'bcm2835_clock_round' [-Wmissing-prototypes]
Reported-by: kernel test robot <lkp at intel.com>

 drivers/clk/bcm/clk-bcm2835.c | 30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/drivers/clk/bcm/clk-bcm2835.c b/drivers/clk/bcm/clk-bcm2835.c
index 3ad20e75fd23..c29b643d1bf5 100644
--- a/drivers/clk/bcm/clk-bcm2835.c
+++ b/drivers/clk/bcm/clk-bcm2835.c
@@ -502,6 +502,8 @@ struct bcm2835_clock_data {
 	bool low_jitter;
 
 	u32 tcnt_mux;
+
+	bool round_up;
 };
 
 struct bcm2835_gate_data {
@@ -992,12 +994,30 @@ static long bcm2835_clock_rate_from_divisor(struct bcm2835_clock *clock,
 	return temp;
 }
 
+static unsigned long bcm2835_clock_round(unsigned long clk)
+{
+	unsigned long scaler;
+
+	/*
+	 * If increasing a clock by less than 0.1% changes it
+	 * from ..999.. to ..000.., round up.
+	 */
+	scaler = 1;
+	while (scaler * 100000 < clk)
+		scaler *= 10;
+	if ((clk + scaler - 1) / scaler % 1000 == 0)
+		clk = (clk / scaler + 1) * scaler;
+
+	return clk;
+}
+
 static unsigned long bcm2835_clock_get_rate(struct clk_hw *hw,
 					    unsigned long parent_rate)
 {
 	struct bcm2835_clock *clock = bcm2835_clock_from_hw(hw);
 	struct bcm2835_cprman *cprman = clock->cprman;
 	const struct bcm2835_clock_data *data = clock->data;
+	unsigned long rate;
 	u32 div;
 
 	if (data->int_bits == 0 && data->frac_bits == 0)
@@ -1005,7 +1025,12 @@ static unsigned long bcm2835_clock_get_rate(struct clk_hw *hw,
 
 	div = cprman_read(cprman, data->div_reg);
 
-	return bcm2835_clock_rate_from_divisor(clock, parent_rate, div);
+	rate = bcm2835_clock_rate_from_divisor(clock, parent_rate, div);
+
+	if (data->round_up)
+		rate = bcm2835_clock_round(rate);
+
+	return rate;
 }
 
 static void bcm2835_clock_wait_busy(struct bcm2835_clock *clock)
@@ -2142,7 +2167,8 @@ static const struct bcm2835_clk_desc clk_desc_array[] = {
 		.div_reg = CM_UARTDIV,
 		.int_bits = 10,
 		.frac_bits = 12,
-		.tcnt_mux = 28),
+		.tcnt_mux = 28,
+		.round_up = true),
 
 	/* TV encoder clock.  Only operating frequency is 108Mhz.  */
 	[BCM2835_CLOCK_VEC]	= REGISTER_PER_CLK(
-- 
2.26.2