octeontx2-pf: tc: fix egress ratelimiting

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>
Link: https://patch.msgid.link/20260717084349.2227796-1-nshettyj@marvell.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/ethernet/marvell/octeontx2/nic/otx2_tc.c b/drivers/net/ethernet/marvell/octeontx2/nic/otx2_tc.c
index 40162b08014dd6562afc3ed049628450d2f537ae..0b46ec29e64eaa14086f99bbffb6030a6097358c 100644 (file)
--- 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;