[PATCH RFC 2/4] clk: rockchip: pll: use round-nearest in determine_rate

[Alexey Charkov]

rockchip_pll_determine_rate() walks the rate table in descending order
and picks the first entry <= the requested rate. This floor-rounding
interacts poorly with consumers that use CLK_SET_RATE_PARENT: a divider
iterating candidates asks the PLL for rate*div, and a tiny undershoot
causes the PLL to snap to a much lower entry.

For example, requesting 1991.04 MHz (248.88 MHz * 8) causes the PLL to
return 1968 MHz instead of 1992 MHz — a 24 MHz table gap that produces
a 1.2% pixel clock error when divided back down.

Change to round-to-nearest: for each table entry compute the absolute
distance from the request, and pick the entry with the smallest delta.
The CCF's divider and composite logic handle over/undershoot preferences
via their own ROUND_CLOSEST flags.

Signed-off-by: Alexey Charkov <alchark at flipper.net>
---
 drivers/clk/rockchip/clk-pll.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/clk/rockchip/clk-pll.c b/drivers/clk/rockchip/clk-pll.c
index 6b853800cb6b..c142f2c4fd99 100644
--- a/drivers/clk/rockchip/clk-pll.c
+++ b/drivers/clk/rockchip/clk-pll.c
@@ -66,19 +66,19 @@ static int rockchip_pll_determine_rate(struct clk_hw *hw,
 {
 	struct rockchip_clk_pll *pll = to_rockchip_clk_pll(hw);
 	const struct rockchip_pll_rate_table *rate_table = pll->rate_table;
+	unsigned long best = 0;
 	int i;
 
-	/* Assuming rate_table is in descending order */
 	for (i = 0; i < pll->rate_count; i++) {
-		if (req->rate >= rate_table[i].rate) {
-			req->rate = rate_table[i].rate;
-
-			return 0;
-		}
+		if (abs((long)req->rate - (long)rate_table[i].rate) <
+		    abs((long)req->rate - (long)best))
+			best = rate_table[i].rate;
 	}
 
-	/* return minimum supported value */
-	req->rate = rate_table[i - 1].rate;
+	if (best)
+		req->rate = best;
+	else
+		req->rate = rate_table[pll->rate_count - 1].rate;
 
 	return 0;
 }

-- 
2.52.0