netdev - Re: [Intel-wired-lan] [PATCH iwl-next v5 07/12] iavf: add support for indirect access to PHC time

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Message-ID: <87o7a6e9gx.fsf@nvidia.com>
Date: Thu, 18 Apr 2024 12:43:00 -0700
From: Rahul Rameshbabu <rrameshbabu@...dia.com>
To: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
Cc: intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
 horms@...nel.org, anthony.l.nguyen@...el.com, Jacob Keller
 <jacob.e.keller@...el.com>, Wojciech Drewek <wojciech.drewek@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH iwl-next v5 07/12] iavf: add support
 for indirect access to PHC time


On Thu, 18 Apr, 2024 01:24:55 -0400 Mateusz Polchlopek <mateusz.polchlopek@...el.com> 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.
>
> Reviewed-by: Wojciech Drewek <wojciech.drewek@...el.com>
> Signed-off-by: Jacob Keller <jacob.e.keller@...el.com>
> Signed-off-by: Mateusz Polchlopek <mateusz.polchlopek@...el.com>
> ---

Reviewed-by: Rahul Rameshbabu <rrameshbabu@...dia.com>