[PATCH] firmware: arm_scmi: Use 64-bit division for clock rate rounding

[PATCH] firmware: arm_scmi: Use 64-bit division for clock rate rounding

[sdunnaga]

From: Steve Dunnagan <sdunnaga@redhat.com>

SCMI clock range descriptors report rates as 64-bit values. When handling
a range clock, scmi_clock_determine_rate() rounds the requested rate up to
the next supported step using the SCMI RATE_STEP value.

The current code uses div64_ul() for this calculation. Since div64_ul()
takes an unsigned long divisor, the 64-bit RATE_STEP value can be truncated
on 32-bit builds. In the worst case, a non-zero 64-bit step can be narrowed
to zero before the division.

Store RATE_STEP in a u64, reject a malformed zero step, and use
DIV64_U64_ROUND_UP() so the divisor is handled as a 64-bit value.

This does not change behavior for valid firmware reporting a non-zero step
that fits in unsigned long.

Tested on Xunlong Orange Pi 5 Plus / RK3588 with SCMI over SMC. SCMI
clocks probed successfully before and after the change. SCMI-backed CPU
clocks were exercised through cpufreq-dt by switching each CPU policy
between its lowest and highest available OPP.

Signed-off-by: Steve Dunnagan <sdunnaga@redhat.com>
---
 drivers/firmware/arm_scmi/clock.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/firmware/arm_scmi/clock.c b/drivers/firmware/arm_scmi/clock.c
index 42e666a628c7..0278705d809e 100644
--- a/drivers/firmware/arm_scmi/clock.c
+++ b/drivers/firmware/arm_scmi/clock.c
@@ -718,7 +718,7 @@ static int scmi_clock_rate_set(const struct scmi_protocol_handle *ph,
 static int scmi_clock_determine_rate(const struct scmi_protocol_handle *ph,
 				     u32 clk_id, unsigned long *rate)
 {
-	u64 fmin, fmax, ftmp;
+	u64 fmin, fmax, ftmp, step;
 	struct scmi_clock_info *clk;
 	struct scmi_clock_desc *clkd;
 	struct clock_info *ci = ph->get_priv(ph);
@@ -749,11 +749,14 @@ static int scmi_clock_determine_rate(const struct scmi_protocol_handle *ph,
 		return 0;
 	}
 
+	step = clkd->r.rates[RATE_STEP];
+	if (!step)
+		return -EINVAL;
+
 	ftmp = *rate - fmin;
-	ftmp += clkd->r.rates[RATE_STEP] - 1; /* to round up */
-	ftmp = div64_ul(ftmp, clkd->r.rates[RATE_STEP]);
+	ftmp = DIV64_U64_ROUND_UP(ftmp, step);
 
-	*rate = ftmp * clkd->r.rates[RATE_STEP] + fmin;
+	*rate = ftmp * step + fmin;
 
 	return 0;
 }
-- 
2.49.0