[PATCH v2] iio: frequency: adf4350: replace loop with fls_long()

[Neel Bullywon <neelb2403@gmail.com>]

Address the TODO in adf4350_set_freq() by replacing the iterative
power-of-2 shift loop with a constant-time bitwise calculation.

By comparing the highest set bits of the target constant and freq
using fls_long(), we can calculate the required RF divider selection
in a single step without relying on expensive 64-bit division.

This ensures freq is properly shifted to meet or exceed the minimum
VCO frequency.

Signed-off-by: Neel Bullywon <neelb2403@gmail.com>
---
Changes in v2:
  - Use fls_long() instead of order_base_2(DIV_ROUND_UP_ULL()) to avoid
    unnecessary 64-bit division (Andy Shevchenko)
  - Add correction check for mantissa edge case
  - Adjust whitespace per review feedback

 drivers/iio/frequency/adf4350.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/drivers/iio/frequency/adf4350.c b/drivers/iio/frequency/adf4350.c
index ed1741165f55..2183deec179d 100644
--- a/drivers/iio/frequency/adf4350.c
+++ b/drivers/iio/frequency/adf4350.c
@@ -17,6 +17,7 @@
 #include <linux/err.h>
 #include <linux/gcd.h>
 #include <linux/gpio/consumer.h>
+#include <linux/bitops.h>
 #include <asm/div64.h>
 #include <linux/clk.h>
 #include <linux/clk-provider.h>
@@ -151,17 +152,14 @@ static int adf4350_set_freq(struct adf4350_state *st, unsigned long long freq)
 
 	st->r4_rf_div_sel = 0;
 
-	/*
-	 * !\TODO: The below computation is making sure we get a power of 2
-	 * shift (st->r4_rf_div_sel) so that freq becomes higher or equal to
-	 * ADF4350_MIN_VCO_FREQ. This might be simplified with fls()/fls_long()
-	 * and friends.
-	 */
-	while (freq < ADF4350_MIN_VCO_FREQ) {
-		freq <<= 1;
-		st->r4_rf_div_sel++;
+	if (freq < ADF4350_MIN_VCO_FREQ) {
+		st->r4_rf_div_sel = fls_long(ADF4350_MIN_VCO_FREQ - 1) - fls_long(freq);
+		if ((freq << st->r4_rf_div_sel) < ADF4350_MIN_VCO_FREQ)
+			st->r4_rf_div_sel++;
 	}
 
+	freq <<= st->r4_rf_div_sel;
+
 	if (freq > ADF4350_MAX_FREQ_45_PRESC) {
 		prescaler = ADF4350_REG1_PRESCALER;
 		mdiv = 75;
-- 
2.44.0