Subject: Re: [PATCH RFC net-next] net: phy: add Marvell PHY PTP support [multicast/DSA issues]

From: Jakub Kicinski <[email-masked]>
Date: Fri, 3 Mar 2023 15:28:55 -0800

On Fri, 3 Mar 2023 14:12:47 +0100 Köry Maincent wrote:
> > Do you happen to have a datasheet for MACB? The time stamping
> > capability is suspiciously saved in a variable called hw_dma_cap
> > which may indicate it's a DMA time stamp not a true PTP stamp.
> > 
> > Quite a few NICs/MACs support DMA time stamps because it's far 
> > easier to take the stamp where the descriptor writing logic is
> > (DMA block) than take it at the end of the MAC and haul it all 
> > the way thru the pipeline back to the DMA block.  
> 
> I don't have the datasheet but indeed you seem to have right. From the support
> commit message: 
> > Time stamps are obtained from the dma buffer descriptors  

That's just saying that the time stamp is communicated via 
a descriptor rather than a register, so not a strong proof 
of where it's taken.

> I suppose it is less precise as using true PTP stamp as it is an hardware block
> further. Is that right?