linux-kernel - [PATCH net-next 6/7] net: stmmac: Fix comment about default addend calculation

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Message-Id: <20230824-stmmac-subsecond-inc-cleanup-v1-6-e0b9f7c18b37@redhat.com>
Date:   Thu, 24 Aug 2023 13:32:57 -0500
From:   Andrew Halaney <ahalaney@...hat.com>
To:     Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Richard Cochran <richardcochran@...il.com>
Cc:     netdev@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Andrew Halaney <ahalaney@...hat.com>
Subject: [PATCH net-next 6/7] net: stmmac: Fix comment about default addend
 calculation

The comment neglects that freq_div_ratio is the ratio between
the subsecond increment frequency and the clk_ptp_rate frequency.

Signed-off-by: Andrew Halaney <ahalaney@...hat.com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
index dfead0df6163..64185753865f 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
@@ -853,10 +853,12 @@ int stmmac_init_tstamp_counter(struct stmmac_priv *priv, u32 systime_flags)
 	/* Store sub second increment for later use */
 	priv->sub_second_inc = sub_second_inc;
 
-	/* calculate default addend value:
-	 * formula is :
-	 * addend = (2^32)/freq_div_ratio;
-	 * where, freq_div_ratio = 1e9ns/sub_second_inc
+	/* Calculate default addend so the accumulator overflows (2^32) in
+	 * sub_second_inc (ns). The addend is added to the accumulator
+	 * every clk_ptp cycle.
+	 *
+	 * addend = (2^32) / freq_div_ratio
+	 * where, freq_div_ratio = (1e9ns / sub_second_inc) / clk_ptp_rate
 	 */
 	temp = div_u64(NSEC_PER_SEC, sub_second_inc);
 	temp = temp << 32;

-- 
2.41.0