linux-kernel - Re: [PATCH iwl-next v9 14/14] igc: add support to get frame preemption statistics via ethtool

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Message-ID: <20250312160052.ypq6u4na3klib6l2@skbuf>
Date: Wed, 12 Mar 2025 18:00:52 +0200
From: Vladimir Oltean <vladimir.oltean@....com>
To: Faizal Rahim <faizal.abdul.rahim@...ux.intel.com>
Cc: Tony Nguyen <anthony.l.nguyen@...el.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Simon Horman <horms@...nel.org>,
	Russell King <linux@...linux.org.uk>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>,
	Furong Xu <0x1207@...il.com>,
	Russell King <rmk+kernel@...linux.org.uk>,
	Serge Semin <fancer.lancer@...il.com>,
	Xiaolei Wang <xiaolei.wang@...driver.com>,
	Suraj Jaiswal <quic_jsuraj@...cinc.com>,
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	Gal Pressman <gal@...dia.com>,
	Jesper Nilsson <jesper.nilsson@...s.com>,
	Choong Yong Liang <yong.liang.choong@...ux.intel.com>,
	Chwee-Lin Choong <chwee.lin.choong@...el.com>,
	Kunihiko Hayashi <hayashi.kunihiko@...ionext.com>,
	Vinicius Costa Gomes <vinicius.gomes@...el.com>,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org
Subject: Re: [PATCH iwl-next v9 14/14] igc: add support to get frame
 preemption statistics via ethtool

On Sun, Mar 09, 2025 at 06:46:48AM -0400, Faizal Rahim wrote:
> +/**
> + * igc_ethtool_get_frame_ass_error - Get the frame assembly error count.
> + * @reg_value: Register value for IGC_PRMEXCPRCNT
> + * Return: The count of frame assembly errors.
> + */
> +static u64 igc_ethtool_get_frame_ass_error(u32 reg_value)
> +{
> +	/* Out of order statistics */
> +	u32 ooo_frame_cnt, ooo_frag_cnt;
> +	u32 miss_frame_frag_cnt;
> +
> +	ooo_frame_cnt = FIELD_GET(IGC_PRMEXCPRCNT_OOO_FRAME_CNT, reg_value);
> +	ooo_frag_cnt = FIELD_GET(IGC_PRMEXCPRCNT_OOO_FRAG_CNT, reg_value);
> +	miss_frame_frag_cnt = FIELD_GET(IGC_PRMEXCPRCNT_MISS_FRAME_FRAG_CNT, reg_value);
> +
> +	return ooo_frame_cnt + ooo_frag_cnt + miss_frame_frag_cnt;
> +}

Counters should be monotonically increasing 64-bit values, and as per your
response in v8, these are 8-bit saturating, clear-on-read values, which
I have some doubts about. But I don't think there's anything that
software can do to make that situation any better. Even reading them
periodically and integrating them into a software sum risks losing
events due to the saturating behavior, which is confusing in its own
way. So I'll have to tolerate this.

Reviewed-by: Vladimir Oltean <vladimir.oltean@....com>