netdev - Re: [PATCH net-next RFC v4 2/5] net: Expose available time stamping layers to user space.

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Message-ID: <1571c8f5-b36a-5af1-d56c-d38f69406840@intel.com>
Date: Wed, 17 May 2023 15:01:34 -0700
From: Jacob Keller <jacob.e.keller@...el.com>
To: Jakub Kicinski <kuba@...nel.org>, Vladimir Oltean
	<vladimir.oltean@....com>
CC: "Russell King (Oracle)" <linux@...linux.org.uk>, Andrew Lunn
	<andrew@...n.ch>, Köry Maincent <kory.maincent@...tlin.com>,
	<netdev@...r.kernel.org>, <glipus@...il.com>,
	<maxime.chevallier@...tlin.com>, <vadim.fedorenko@...ux.dev>,
	<richardcochran@...il.com>, <gerhard@...leder-embedded.com>,
	<thomas.petazzoni@...tlin.com>, <krzysztof.kozlowski+dt@...aro.org>,
	<robh+dt@...nel.org>
Subject: Re: [PATCH net-next RFC v4 2/5] net: Expose available time stamping
 layers to user space.

On 5/11/2023 4:16 PM, Jakub Kicinski wrote:
> On Fri, 12 May 2023 02:07:17 +0300 Vladimir Oltean wrote:
>> AFAIU from igc_ptp_tx_hwtstamp(), it's just that the igc DMA
>> controller did not bother to transport the timestamps from the MAC
>> back into the descriptor, leaving it up to software to do it out of
>> band, which of course may cause correlation bugs and limits
>> throughput. Surely they can do better.
> 

In myy understanding for for MAC timestamping, the igc hardware stores
the timestamp in a register and assumes you only have one outstanding
timestamp request at a time.

The MAC timestamp is captured well after the Tx descriptor is written
back as complete. There's no completion queue on this device to send
such a message out-of-band of the Tx writeback. Delaying the writeback
until after the Tx timestamp is captured would have its own challenges.

Obviously it *can* be done better than this, but thats not how the igc
hardware was designed.