netdev - Re: [PATCH 2/2] net: intel: igb: add RR2DCDELAY to ethtool registers dump

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Date:   Fri, 21 Jun 2019 01:47:50 +0000
From:   "Brown, Aaron F" <aaron.f.brown@...el.com>
To:     Artem Bityutskiy <dedekind1@...il.com>,
        "David S. Miller" <davem@...emloft.net>
CC:     "Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
        "Fujinaka, Todd" <todd.fujinaka@...el.com>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/2] net: intel: igb: add RR2DCDELAY to ethtool
 registers dump

On Tue, 2019-06-18 at 14:55 +0300, Artem Bityutskiy wrote:
> From: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> 
> This patch adds the RR2DCDELAY register to the ethtool registers dump.
> RR2DCDELAY exists on I210 and I211 Intel Gigabit Ethernet chips and it stands
> for "Read Request To Data Completion Delay". Here is how this register is
> described in the I210 datasheet:
> 
> "This field captures the maximum PCIe split time in 16 ns units, which is the
> maximum delay between the read request to the first data completion. This is
> giving an estimation of the PCIe round trip time."
> 
> In other words, whenever I210 reads from the host memory (e.g., fetches a
> descriptor from the ring), the chip measures every PCI DMA read transaction and
> captures the maximum value. So it ends up containing the longest DMA
> transaction time.
> 
> This register is very useful for troubleshooting and research purposes. If you
> are dealing with time-sensitive networks, this register can help you get
> an idea of your "I210-to-ring" latency. This helps answering questions like
> "should I have PCIe ASPM enabled?" or "should I enable deep C-states?" on
> my system.
> 
> It is safe to read this register at any point, reading it has no effect on
> the I210 chip functionality.
> 
> Signed-off-by: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> ---
>  drivers/net/ethernet/intel/igb/e1000_regs.h  | 2 ++
>  drivers/net/ethernet/intel/igb/igb_ethtool.c | 5 ++++-
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 

Tested-by: Aaron Brown <aaron.f.brown@...el.com>