patch "iio: frequency: adf4350: Fix prescaler usage." added to char-misc-testing

patch "iio: frequency: adf4350: Fix prescaler usage." added to char-misc-testing

[gregkh]

This is a note to let you know that I've just added the patch titled

    iio: frequency: adf4350: Fix prescaler usage.

to my char-misc git tree which can be found at
    git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/char-misc.git
in the char-misc-testing branch.

The patch will show up in the next release of the linux-next tree
(usually sometime within the next 24 hours during the week.)

The patch will be merged to the char-misc-next branch sometime soon,
after it passes testing, and the merge window is open.

If you have any questions about this process, please let me know.


From 33d7ecbf69aa7dd4145e3b77962bcb8759eede3d Mon Sep 17 00:00:00 2001
From: Michael Hennerich <michael.hennerich@analog.com>
Date: Fri, 29 Aug 2025 12:25:42 +0100
Subject: iio: frequency: adf4350: Fix prescaler usage.
MIME-Version: 1.0
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Content-Transfer-Encoding: 8bit

The ADF4350/1 features a programmable dual-modulus prescaler of 4/5 or 8/9.
When set to 4/5, the maximum RF frequency allowed is 3 GHz.
Therefore, when operating the ADF4351 above 3 GHz, this must be set to 8/9.
In this context not the RF output frequency is meant
- it's the VCO frequency.

Therefore move the prescaler selection after we derived the VCO frequency
from the desired RF output frequency.

This BUG may have caused PLL lock instabilities when operating the VCO at
the very high range close to 4.4 GHz.

Fixes: e31166f0fd48 ("iio: frequency: New driver for Analog Devices ADF4350/ADF4351 Wideband Synthesizers")
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/20250829-adf4350-fix-v2-1-0bf543ba797d@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
---
 drivers/iio/frequency/adf4350.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/drivers/iio/frequency/adf4350.c b/drivers/iio/frequency/adf4350.c
index 47f1c7e9efa9..475a7a653bfb 100644
--- a/drivers/iio/frequency/adf4350.c
+++ b/drivers/iio/frequency/adf4350.c
@@ -149,6 +149,19 @@ static int adf4350_set_freq(struct adf4350_state *st, unsigned long long freq)
 	if (freq > ADF4350_MAX_OUT_FREQ || freq < st->min_out_freq)
 		return -EINVAL;
 
+	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_MAX_FREQ_45_PRESC) {
 		prescaler = ADF4350_REG1_PRESCALER;
 		mdiv = 75;
@@ -157,13 +170,6 @@ static int adf4350_set_freq(struct adf4350_state *st, unsigned long long freq)
 		mdiv = 23;
 	}
 
-	st->r4_rf_div_sel = 0;
-
-	while (freq < ADF4350_MIN_VCO_FREQ) {
-		freq <<= 1;
-		st->r4_rf_div_sel++;
-	}
-
 	/*
 	 * Allow a predefined reference division factor
 	 * if not set, compute our own
-- 
2.51.0