[PATCH v4] staging: iio: frequency: ad9832/ad9834: add comment explaining do_div usage

[Mohamad Raizudeen]

Previous attempts to change do_div() to div64_ul were rejected because
mclk will always fit within 32 fits for this hardware, making do_div()
safe to use.

However, Coccinelle continues to flag this as a false positive. To
prevent future developers from submitting unnecessary fixes, add a
comment explaining why do_div() is intentionally kept.

Suggested-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
Signed-off-by: Mohamad Raizudeen <raizudeen.kerneldev@xxxxxxxxx>
---
Changes in v4:
- Fixed typo in comment (fir -> fit) as pointed out by Joshua Crofts.

 drivers/staging/iio/frequency/ad9832.c | 6 ++++++
 drivers/staging/iio/frequency/ad9834.c | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/drivers/staging/iio/frequency/ad9832.c b/drivers/staging/iio/frequency/ad9832.c
index 60c33e10c46f..2e216c950c53 100644
--- a/drivers/staging/iio/frequency/ad9832.c
+++ b/drivers/staging/iio/frequency/ad9832.c
@@ -118,6 +118,12 @@ static unsigned long ad9832_calc_freqreg(unsigned long mclk, unsigned long fout)
 {
 	u64 freqreg = (u64)fout << AD9832_FREQ_BITS;
 
+	/*
+	 * mclk is an unsigned long, which triggers a Coccinelle false positive
+	 * warning about using do_div() for 64-by-32 division. However, mclk
+	 * for this hardware will always fit within 32 bits, so do_div() is
+	 * safe to use here.
+	 */
 	do_div(freqreg, mclk);
 	return freqreg;
 }
diff --git a/drivers/staging/iio/frequency/ad9834.c b/drivers/staging/iio/frequency/ad9834.c
index 33dfd723923c..e089a94db97d 100644
--- a/drivers/staging/iio/frequency/ad9834.c
+++ b/drivers/staging/iio/frequency/ad9834.c
@@ -103,6 +103,12 @@ static unsigned int ad9834_calc_freqreg(unsigned long mclk, unsigned long fout)
 {
 	unsigned long long freqreg = (u64)fout * (u64)BIT(AD9834_FREQ_BITS);
 
+	/*
+	 * mclk is an unsigned long, which triggers a Coccinelle false positive
+	 * warning about using a do_div() for 64-by-32 division. However, mclk
+	 * for this hardware will always fit within 32 bits, so do_div() is
+	 * safe to use here.
+	 */
 	do_div(freqreg, mclk);
 	return freqreg;
 }
-- 
2.53.0