Subject: Re: [PATCH net-next v5 08/16] net: ethtool: Add a command to expose current time stamping layer

From: Jakub Kicinski <[email-masked]>
Date: Fri, 13 Oct 2023 10:46:06 -0700

On Fri, 13 Oct 2023 20:09:03 +0300 Vladimir Oltean wrote:
> > > Translation between the UAPI-visible PHC index and MAC, DMA, phylib
> > > PHY, other PHY etc can then be done by the kernel as needed.  
> > 
> > Translation by the kernel at which point?  
> 
> The gist of what I'm proposing is for the core ethtool netlink message
> handler to get just the phc_index as an attribute. No other information
> as to what it represents. Not that it's netdev, DMA, phylib PHY or whatnot.
> 
> The ethtool kernel code would iterate through the stuff registered in
> the system for the netdev, calling get_ts_info() or phy_ts_info() on it,
> until it finds something which populates struct ethtool_ts_info ::
> phc_index with the phc_index retrieved from netlink.
> 
> Then, ethtool just talks with the timestamper that matched that phc_index.
> 
> Same idea would be applied for the command that lists all timestamping
> layers for a netdev. Check get_ts_info(), phy_ts_info(dev->phydev), and
> can be extended in the future.

I see, that could work. The user would then dig around sysfs to figure
out which PHC has what characteristics?

> > IMHO it'd indeed be clearer for the user to have an ability to read 
> > the PHC for SOF_..._DMA via ETHTOOL_MSG_TS_LIST_GET_REPLY as a separate
> > entry, rather than e.g. assume that DMA uses the same PHC as MAC.  
> 
> I'm not really sure what you're referring to, with SOF_..._DMA.
> The DMA, if presented as a PHC as I am proposing, would play the role of
> the hardware timestamp provider (think SOF_TIMESTAMPING_TX_HARDWARE |
> SOF_TIMESTAMPING_RX_HARDWARE), so there will be no driver-visible
> special socket option flags for DMA timestamping.

Each packet may want different timestamp tho, especially on Tx it
should be fairly easy for socket to request to get "real" MAC stamps,
while most get cheaper DMA stamps. Currently some drivers run flow
matching to find PTP packets and automatically give them better quality
timestamps :(

Even if at the config level we use PHCs we need to translate that into
some SKBTX_* bit, don't we?