linux-kernel - [RFC PATCH 1/3] clk: sunxi-ng: nkm: Minimize difference when finding rate

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Date:   Sat, 27 May 2023 15:27:45 +0200
From:   Frank Oltmanns <frank@...manns.dev>
To:     linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-sunxi@...ts.linux.dev
Cc:     Frank Oltmanns <frank@...manns.dev>,
        Andre Przywara <andre.przywara@....com>,
        Chen-Yu Tsai <wens@...e.org>, Icenowy Zheng <icenowy@...c.io>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Maxime Ripard <mripard@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Rob Herring <robh@...nel.org>,
        Samuel Holland <samuel@...lland.org>,
        Stephen Boyd <sboyd@...nel.org>
Subject: [RFC PATCH 1/3] clk: sunxi-ng: nkm: Minimize difference when finding rate

Instead of selecting the first rate that is less than the requested
rate, select the rate that minimizes the absolute difference with the
requested rate. This ensures a more accurate rate selection, when the
closest available rate is higher than the requested rate.

Also stop searching once an exact match is found.

Adjust mux to this change in rate selection.

Signed-off-by: Frank Oltmanns <frank@...manns.dev>
---
 drivers/clk/sunxi-ng/ccu_mux.c |  2 +-
 drivers/clk/sunxi-ng/ccu_nkm.c | 13 +++++++++----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/clk/sunxi-ng/ccu_mux.c b/drivers/clk/sunxi-ng/ccu_mux.c
index 9303e1f02ffd..6fe0a8d098f1 100644
--- a/drivers/clk/sunxi-ng/ccu_mux.c
+++ b/drivers/clk/sunxi-ng/ccu_mux.c
@@ -145,7 +145,7 @@ int ccu_mux_helper_determine_rate(struct ccu_common *common,
 			goto out;
 		}
 
-		if ((req->rate - tmp_rate) < (req->rate - best_rate)) {
+		if (abs(req->rate - tmp_rate) < abs(req->rate - best_rate)) {
 			best_rate = tmp_rate;
 			best_parent_rate = parent_rate;
 			best_parent = parent;
diff --git a/drivers/clk/sunxi-ng/ccu_nkm.c b/drivers/clk/sunxi-ng/ccu_nkm.c
index a0978a50edae..94d2a83992b2 100644
--- a/drivers/clk/sunxi-ng/ccu_nkm.c
+++ b/drivers/clk/sunxi-ng/ccu_nkm.c
@@ -19,7 +19,7 @@ struct _ccu_nkm {
 static unsigned long ccu_nkm_find_best(unsigned long parent, unsigned long rate,
 				       struct _ccu_nkm *nkm)
 {
-	unsigned long best_rate = 0;
+	unsigned long best_rate = 0, best_diff = ULONG_MAX;
 	unsigned long best_n = 0, best_k = 0, best_m = 0;
 	unsigned long _n, _k, _m;
 
@@ -27,21 +27,26 @@ static unsigned long ccu_nkm_find_best(unsigned long parent, unsigned long rate,
 		for (_n = nkm->min_n; _n <= nkm->max_n; _n++) {
 			for (_m = nkm->min_m; _m <= nkm->max_m; _m++) {
 				unsigned long tmp_rate;
+				unsigned long tmp_diff;
 
 				tmp_rate = parent * _n * _k / _m;
 
-				if (tmp_rate > rate)
-					continue;
-				if ((rate - tmp_rate) < (rate - best_rate)) {
+				tmp_diff = abs(rate - tmp_rate);
+
+				if (tmp_diff < best_diff) {
 					best_rate = tmp_rate;
+					best_diff = tmp_diff;
 					best_n = _n;
 					best_k = _k;
 					best_m = _m;
+					if (best_diff == 0)
+						goto out;
 				}
 			}
 		}
 	}
 
+out:
 	nkm->n = best_n;
 	nkm->k = best_k;
 	nkm->m = best_m;
-- 
2.40.1