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

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Date:   Fri, 26 Feb 2021 01:00:26 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Michael Walle <michael@...le.cc>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandru Marginean <alexandru.marginean@....com>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: 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:
> > @@ -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.