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

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Date: Tue, 11 Jun 2024 12:43:45 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Andrew Lunn <andrew@...n.ch>
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 6/10/24 20:29, Andrew Lunn wrote:
> 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.

Yes, but then our stats remain stale forever, because we don't refresh
stats before reading them as detailed in my other response.

--Sean