net/sfc/base: clarify meaning of Rx desc lbits in PS mode

There is no point to add mask plus one before mask applying since it
still does not help to avoid overflow on subtract. Modulo mask
arithmetic works perfectly for unsigned integers of the same type.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>

diff --git a/drivers/net/sfc/base/ef10_ev.c b/drivers/net/sfc/base/ef10_ev.c
index eb9ec759acbc59936c5def38d2a4f6987a0d76a4..9ed22f8aa4ffd2a4fc8e5c60102b0f51feb89c89 100644 (file)
--- a/drivers/net/sfc/base/ef10_ev.c
+++ b/drivers/net/sfc/base/ef10_ev.c
@@ -774,7 +774,7 @@ ef10_ev_rx_packed_stream(
        __in_opt        void *arg)
 {
        uint32_t label;
-       uint32_t next_read_lbits;
+       uint32_t pkt_count_lbits;
        uint16_t flags;
        boolean_t should_abort;
        efx_evq_rxq_state_t *eersp;
@@ -782,15 +782,22 @@ ef10_ev_rx_packed_stream(
        unsigned int current_id;
        boolean_t new_buffer;
 
-       next_read_lbits = EFX_QWORD_FIELD(*eqp, ESF_DZ_RX_DSC_PTR_LBITS);
+       pkt_count_lbits = EFX_QWORD_FIELD(*eqp, ESF_DZ_RX_DSC_PTR_LBITS);
        label = EFX_QWORD_FIELD(*eqp, ESF_DZ_RX_QLABEL);
        new_buffer = EFX_QWORD_FIELD(*eqp, ESF_DZ_RX_EV_ROTATE);
 
        flags = 0;
 
        eersp = &eep->ee_rxq_state[label];
-       pkt_count = (EFX_MASK32(ESF_DZ_RX_DSC_PTR_LBITS) + 1 +
-           next_read_lbits - eersp->eers_rx_stream_npackets) &
+
+       /*
+        * RX_DSC_PTR_LBITS has least significant bits of the global
+        * (not per-buffer) packet counter. It is guaranteed that
+        * maximum number of completed packets fits in lbits-mask.
+        * So, modulo lbits-mask arithmetic should be used to calculate
+        * packet counter increment.
+        */
+       pkt_count = (pkt_count_lbits - eersp->eers_rx_stream_npackets) &
            EFX_MASK32(ESF_DZ_RX_DSC_PTR_LBITS);
        eersp->eers_rx_stream_npackets += pkt_count;