netdev - Re: [PATCH net-next 4/5] eth: fbnic: add PCIe hardware statistics

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Message-ID: <20241118073501.4e44d114@kernel.org>
Date: Mon, 18 Nov 2024 07:35:01 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Andrew Lunn <andrew@...n.ch>
Cc: davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com,
 pabeni@...hat.com, alexanderduyck@...com, Sanman Pradhan
 <sanman.p211993@...il.com>
Subject: Re: [PATCH net-next 4/5] eth: fbnic: add PCIe hardware statistics

On Sun, 17 Nov 2024 21:19:00 +0100 Andrew Lunn wrote:
> > +/* PUL User Registers*/
> > +#define FBNIC_PUL_USER_OB_RD_TLP_CNT_31_0 \
> > +					0x3106e		/* 0xc41b8 */  
> 
> Is there a comment somewhere which explains what these comments mean?
> Otherwise they appear to be useless magic numbers.

The number on the left is the number on the right divided by 4.
We index the registers as an array of u32s so the define is an index,
but for grep-ability we add the absolute address in the comment.

> > +static void fbnic_hw_stat_rst64(struct fbnic_dev *fbd, u32 reg, s32 offset,
> > +				struct fbnic_stat_counter *stat)
> > +{
> > +	/* Record initial counter values and compute deltas from there to ensure
> > +	 * stats start at 0 after reboot/reset. This avoids exposing absolute
> > +	 * hardware counter values to userspace.
> > +	 */
> > +	stat->u.old_reg_value_64 = fbnic_stat_rd64(fbd, reg, offset);  
> 
> I don't know how you use these stats, but now they are in debugfs, do
> they actually need to be 0 after reboot/reset? For most debugging
> performance issues with statistics, i want to know how much a counter
> goes up per second, so userspace will be reading the values twice with
> a sleep, and doing a subtractions anyway. So why not make the kernel
> code simpler?

Right, there is no strong reason to reset debugfs stats. OTOH
consistent behavior across all stats is nice (from rtnl stats 
to debugfs). And we will need the reset helpers for other stats.
Meta uses a lot of multi-host systems, the NIC is reset much
less often than the machines. Starting at 0 is convenient for 
manual debug.