netdev - Re: [Intel-wired-lan] [PATCH iwl-next v7 07/12] iavf: add support for indirect access to PHC time

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Date: Sat, 8 Jun 2024 13:59:04 +0100
From: Simon Horman <horms@...nel.org>
To: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
Cc: intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	Jacob Keller <jacob.e.keller@...el.com>,
	Wojciech Drewek <wojciech.drewek@...el.com>,
	Rahul Rameshbabu <rrameshbabu@...dia.com>
Subject: Re: [Intel-wired-lan] [PATCH iwl-next v7 07/12] iavf: add support
 for indirect access to PHC time

On Tue, Jun 04, 2024 at 09:13:55AM -0400, Mateusz Polchlopek wrote:
> From: Jacob Keller <jacob.e.keller@...el.com>
> 
> Implement support for reading the PHC time indirectly via the
> VIRTCHNL_OP_1588_PTP_GET_TIME operation.
> 
> Based on some simple tests with ftrace, the latency of the indirect
> clock access appears to be about ~110 microseconds. This is due to the
> cost of preparing a message to send over the virtchnl queue.
> 
> This is expected, due to the increased jitter caused by sending messages
> over virtchnl. It is not easy to control the precise time that the
> message is sent by the VF, or the time that the message is responded to
> by the PF, or the time that the message sent from the PF is received by
> the VF.
> 
> For sending the request, note that many PTP related operations will
> require sending of VIRTCHNL messages. Instead of adding a separate AQ
> flag and storage for each operation, setup a simple queue mechanism for
> queuing up virtchnl messages.
> 
> Each message will be converted to a iavf_ptp_aq_cmd structure which ends
> with a flexible array member. A single AQ flag is added for processing
> messages from this queue. In principle this could be extended to handle
> arbitrary virtchnl messages. For now it is kept to PTP-specific as the
> need is primarily for handling PTP-related commands.
> 
> Use this to implement .gettimex64 using the indirect method via the
> virtchnl command. The response from the PF is processed and stored into
> the cached_phc_time. A wait queue is used to allow the PTP clock gettime
> request to sleep until the message is sent from the PF.
> 
> Signed-off-by: Jacob Keller <jacob.e.keller@...el.com>
> Reviewed-by: Wojciech Drewek <wojciech.drewek@...el.com>
> Reviewed-by: Rahul Rameshbabu <rrameshbabu@...dia.com>
> Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@...el.com>

Reviewed-by: Simon Horman <horms@...nel.org>