netdev - Re: [PATCH v2] ethtool: igb: dump RR2DCDELAY register

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Date:   Mon, 15 Jul 2019 21:26:46 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     netdev@...r.kernel.org
Cc:     Artem Bityutskiy <dedekind1@...il.com>,
        "John W . Linville" <linville@...driver.com>
Subject: Re: [PATCH v2] ethtool: igb: dump RR2DCDELAY register

On Mon, Jul 15, 2019 at 01:59:33PM +0300, Artem Bityutskiy wrote:
> From: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> 
> Decode 'RR2DCDELAY' register which Linux kernel provides starting from version
> 5.3. The corresponding commit in the Linux kernel is:
>     cd502a7f7c9c igb: add RR2DCDELAY to ethtool registers dump
> 
> The RR2DCDELAY register is present in 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 practice, this register can be used to measure the time it takes the NIC to
> read data from the host memory.
> 
> Signed-off-by: Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>
> ---
>  igb.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> Changelog:
> v2: Fixed a typo in the commentary.
> v1: Initial pach version.
> 
> 
> diff --git a/igb.c b/igb.c
> index e0ccef9..cb24877 100644
> --- a/igb.c
> +++ b/igb.c
> @@ -859,6 +859,18 @@ igb_dump_regs(struct ethtool_drvinfo *info, struct ethtool_regs *regs)
>  		"0x03430: TDFPC       (Tx data FIFO packet count)      0x%08X\n",
>  		regs_buff[550]);
>  
> +	/*
> +	 * Starting from kernel version 5.3 the registers dump buffer grew from
> +	 * 739 4-byte words to 740 words, and word 740 contains the RR2DCDELAY
> +	 * register.
> +	 */
> +	if (regs->len < 740)
> +		return 0;
> +
> +	fprintf(stdout,
> +		"0x05BF4: RR2DCDELAY  (Max. DMA read delay)            0x%08X\n",
> +		regs_buff[739]);
> +
>  	return 0;
>  }
>  

Reviewed-by: Michal Kubecek <mkubecek@...e.cz>