linux-kernel - [PATCH] clk: analogbits: Fix incorrect calculation of vco rate delta

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Message-Id: <20240827061954.351773-1-ganboing@gmail.com>
Date: Mon, 26 Aug 2024 23:19:54 -0700
From: Bo Gan <ganboing@...il.com>
To: linux-clk@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: samuel.holland@...ive.com,
	emil.renner.berthing@...onical.com,
	mturquette@...libre.com,
	paul.walmsley@...ive.com,
	sboyd@...nel.org
Subject: [PATCH] clk: analogbits: Fix incorrect calculation of vco rate delta

In function `wrpll_configure_for_rate`, we try to determine the best PLL
configuration for a target rate. However, in the loop where we try values
of R, we should compare the derived `vco` with `target_vco_rate`. However,
we were in fact comparing it with `target_rate`, which is actually after
Q shift. This is incorrect, and sometimes can result in suboptimal clock
rates. This patch fixes it.

Signed-off-by: Bo Gan <ganboing@...il.com>
---
 drivers/clk/analogbits/wrpll-cln28hpc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/clk/analogbits/wrpll-cln28hpc.c b/drivers/clk/analogbits/wrpll-cln28hpc.c
index 65d422a588e1..9d178afc73bd 100644
--- a/drivers/clk/analogbits/wrpll-cln28hpc.c
+++ b/drivers/clk/analogbits/wrpll-cln28hpc.c
@@ -255,81 +255,81 @@ int wrpll_configure_for_rate(struct wrpll_cfg *c, u32 target_rate,
 	}
 
 	c->flags &= ~WRPLL_FLAGS_BYPASS_MASK;
 
 	/* Calculate the Q shift and target VCO rate */
 	divq = __wrpll_calc_divq(target_rate, &target_vco_rate);
 	if (!divq)
 		return -1;
 	c->divq = divq;
 
 	/* Precalculate the pre-Q divider target ratio */
 	ratio = div64_u64((target_vco_rate << ROUND_SHIFT), parent_rate);
 
 	fbdiv = __wrpll_calc_fbdiv(c);
 	best_r = 0;
 	best_f = 0;
 	best_delta = MAX_VCO_FREQ;
 
 	/*
 	 * Consider all values for R which land within
 	 * [MIN_POST_DIVR_FREQ, MAX_POST_DIVR_FREQ]; prefer smaller R
 	 */
 	for (r = c->init_r; r <= c->max_r; ++r) {
 		f_pre_div = ratio * r;
 		f = (f_pre_div + (1 << ROUND_SHIFT)) >> ROUND_SHIFT;
 		f >>= (fbdiv - 1);
 
 		post_divr_freq = div_u64(parent_rate, r);
 		vco_pre = fbdiv * post_divr_freq;
 		vco = vco_pre * f;
 
 		/* Ensure rounding didn't take us out of range */
 		if (vco > target_vco_rate) {
 			--f;
 			vco = vco_pre * f;
 		} else if (vco < MIN_VCO_FREQ) {
 			++f;
 			vco = vco_pre * f;
 		}
 
-		delta = abs(target_rate - vco);
+		delta = abs(target_vco_rate - vco);
 		if (delta < best_delta) {
 			best_delta = delta;
 			best_r = r;
 			best_f = f;
 		}
 	}
 
 	c->divr = best_r - 1;
 	c->divf = best_f - 1;
 
 	post_divr_freq = div_u64(parent_rate, best_r);
 
 	/* Pick the best PLL jitter filter */
 	range = __wrpll_calc_filter_range(post_divr_freq);
 	if (range < 0)
 		return range;
 	c->range = range;
 
 	return 0;
 }
 EXPORT_SYMBOL_GPL(wrpll_configure_for_rate);
 
 /**
  * wrpll_calc_output_rate() - calculate the PLL's target output rate
  * @c: ptr to a struct wrpll_cfg record to read from
  * @parent_rate: PLL refclk rate
  *
  * Given a pointer to the PLL's current input configuration @c and the
  * PLL's input reference clock rate @parent_rate (before the R
  * pre-divider), calculate the PLL's output clock rate (after the Q
  * post-divider).
  *
  * Context: Any context.  Caller must protect the memory pointed to by @c
  *          from simultaneous modification.
  *
  * Return: the PLL's output clock rate, in Hz.  The return value from
  *         this function is intended to be convenient to pass directly
  *         to the Linux clock framework; thus there is no explicit
  *         error return value.
  */
-- 
2.34.1