linux-kernel - Re: [PATCH net-next 3/3] net: xilinx: axienet: Add statistics support

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Date: Tue, 11 Jun 2024 02:29:05 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Sean Anderson <sean.anderson@...ux.dev>
Cc: Radhey Shyam Pandey <radhey.shyam.pandey@....com>,
	netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Michal Simek <michal.simek@....com>,
	Jakub Kicinski <kuba@...nel.org>,
	Russell King <linux@...linux.org.uk>,
	Paolo Abeni <pabeni@...hat.com>, Eric Dumazet <edumazet@...gle.com>,
	linux-kernel@...r.kernel.org,
	"David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH net-next 3/3] net: xilinx: axienet: Add statistics support

On Tue, Jun 11, 2024 at 02:13:40AM +0200, Andrew Lunn wrote:
> On Mon, Jun 10, 2024 at 07:10:22PM -0400, Sean Anderson wrote:
> > Add support for reading the statistics counters, if they are enabled.
> > The counters may be 64-bit, but we can't detect this as there's no
> > ability bit for it and the counters are read-only. Therefore, we assume
> > the counters are 32-bits.
> 
> > +static void axienet_stats_update(struct axienet_local *lp)
> > +{
> > +	enum temac_stat stat;
> > +
> > +	lockdep_assert_held(&lp->stats_lock);
> > +
> > +	u64_stats_update_begin(&lp->hw_stat_sync);
> > +	for (stat = 0; stat < STAT_COUNT; stat++) {
> > +		u32 counter = axienet_ior(lp, XAE_STATS_OFFSET + stat * 8);
> 
> The * 8 here suggests the counters are spaced so that they could be 64
> bit wide, even when only 32 bits are used. Does the documentation say
> anything about the upper 32 bits when the counters are only 32 bits?
> Are they guaranteed to read as zero? I'm just wondering if the code
> should be forward looking and read all 64 bits? 

Actually, if you read the upper 32 bits and they are not 0, you know
you have 64 bit counters. You can then kill off your period task, it
is not needed because your software counters will wrap around the same
time as the hardware counters.

     Andrew