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

Jacob Keller
2023-05-17T22:01:44Z

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.