[PATCH net] octeontx2-pf: tc: fix egress ratelimiting

[<nshettyj@marvell.com>]

From: Hariprasad Kelam <hkelam@marvell.com>

The egress rate calculation computes an incorrect mantissa and exponent,
causing up to ~50% deviation from the configured rate at lower speeds.

Rework the computation to follow the hardware rate formula:

	rate = 2 * (1 + mantissa/256) * 2^exp / (1 << div_exp)

Keep div_exp = 0 and derive exp and mantissa from half of the requested
rate. Rates below 2 Mbps are floored to the smallest encodable step
(exp = 0, mantissa = 0).

Fixes: e638a83f167e ("octeontx2-pf: TC_MATCHALL egress ratelimiting offload")
Signed-off-by: Hariprasad Kelam <hkelam@marvell.com>
Signed-off-by: Nitin Shetty J <nshettyj@marvell.com>
---
 .../ethernet/marvell/octeontx2/nic/otx2_tc.c  | 29 ++++++++++---------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/drivers/net/ethernet/marvell/octeontx2/nic/otx2_tc.c b/drivers/net/ethernet/marvell/octeontx2/nic/otx2_tc.c
index 40162b08014d..0b46ec29e64e 100644
--- a/drivers/net/ethernet/marvell/octeontx2/nic/otx2_tc.c
+++ b/drivers/net/ethernet/marvell/octeontx2/nic/otx2_tc.c
@@ -30,6 +30,7 @@
 #define OTX2_UNSUPP_LSE_DEPTH		GENMASK(6, 4)
 
 #define MCAST_INVALID_GRP		(-1U)
+#define RATE_MANTISSA_BITS		8
 
 static void otx2_get_egress_burst_cfg(struct otx2_nic *nic, u32 burst,
 				      u32 *burst_exp, u32 *burst_mantissa)
@@ -66,28 +67,30 @@ static void otx2_get_egress_burst_cfg(struct otx2_nic *nic, u32 burst,
 static void otx2_get_egress_rate_cfg(u64 maxrate, u32 *exp,
 				     u32 *mantissa, u32 *div_exp)
 {
-	u64 tmp;
-
 	/* Rate calculation by hardware
 	 *
 	 * PIR_ADD = ((256 + mantissa) << exp) / 256
 	 * rate = (2 * PIR_ADD) / ( 1 << div_exp)
 	 * The resultant rate is in Mbps.
+	 *
+	 * Use div_exp = 0 and compute exp/mantissa for maxrate / 2; the
+	 * leading factor of two yields the full rate. Rates below 2 Mbps
+	 * are floored to the smallest step (exp = 0, mantissa = 0).
 	 */
 
-	/* 2Mbps to 100Gbps can be expressed with div_exp = 0.
-	 * Setting this to '0' will ease the calculation of
-	 * exponent and mantissa.
-	 */
 	*div_exp = 0;
-
 	if (maxrate) {
-		*exp = ilog2(maxrate) ? ilog2(maxrate) - 1 : 0;
-		tmp = maxrate - rounddown_pow_of_two(maxrate);
-		if (maxrate < MAX_RATE_MANTISSA)
-			*mantissa = tmp * 2;
-		else
-			*mantissa = tmp / (1ULL << (*exp - 7));
+		maxrate = maxrate / 2;
+		if (!maxrate) {
+			/* Rates below 2 Mbps map to the smallest step */
+			*exp = 0;
+			*mantissa = 0;
+		} else {
+			*exp = ilog2(maxrate);
+			/* Clear MSB and derive fractional bits */
+			maxrate &= ~BIT(*exp);
+			*mantissa = (maxrate << RATE_MANTISSA_BITS) >> *exp;
+		}
 	} else {
 		/* Instead of disabling rate limiting, set all values to max */
 		*exp = MAX_RATE_EXPONENT;
-- 
2.48.1