Re: [PATCH v2 net 3/6] net: enetc: take the MDIO lock only once per NAPI poll cycle

On Thu, Feb 25, 2021 at 11:52:19PM +0100, Andrew Lunn wrote:

On Thu, Feb 25, 2021 at 02:18:32PM +0200, Vladimir Oltean wrote:

quoted

@@ -327,8 +329,8 @@ static void enetc_get_tx_tstamp(struct enetc_hw *hw, union enetc_tx_bd *txbd,
 {
 	u32 lo, hi, tstamp_lo;
 
-	lo = enetc_rd(hw, ENETC_SICTR0);
-	hi = enetc_rd(hw, ENETC_SICTR1);
+	lo = enetc_rd_hot(hw, ENETC_SICTR0);
+	hi = enetc_rd_hot(hw, ENETC_SICTR1);
 	tstamp_lo = le32_to_cpu(txbd->wb.tstamp);
 	if (lo <= tstamp_lo)
 		hi -= 1;

Hi Vladimir

This change is not obvious, and there is no mention of it in the
commit message. Please could you explain it. I guess it is to do with
enetc_get_tx_tstamp() being called with the MDIO lock held now, when
it was not before?

I realize this is an uncharacteristically short commit message and I'm
sorry for that, if needed I can resend.

Your assumption is correct, the new call path is:

enetc_msix
-> napi_schedule
   -> enetc_poll
      -> enetc_lock_mdio
      -> enetc_clean_tx_ring
         -> enetc_get_tx_tstamp
      -> enetc_clean_rx_ring
      -> enetc_unlock_mdio

The 'hot' accessors are for normal, 'unlocked' register reads and
writes, while enetc_rd contains enetc_lock_mdio, followed by the actual
read, followed by enetc_unlock_mdio.

The goal is to eventually get rid of all the _hot stuff and always take
the lock from the top level, this would allow us to do more register
read/write batching and that would amortize the cost of the locking
overall.