Re: [PATCH net-next] gve: Correctly report software timestamping capabilities

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Jakub Kicinski wrote:
> On Tue, 09 Apr 2024 21:40:46 -0700 Rahul Rameshbabu wrote:
> > > My gut tells me we force drivers to set the ethtool op because
> > > while at it they will probably also implement tx stamping.  
> > 
> > I think the logic should be the other way (in terms of the
> > relationship). A call to skb_tx_timestamp should throw a warning if the
> > driver does not advertise its timestamping capabilities. This way, a
> > naive netdev driver for some lightweight device does not need to worry
> > about this. I agree that anyone implementing tx timestamping should have
> > this operation defined. An skb does not contain any mechanism to
> > reference the driver's ethtool callback. Maybe the right choice is to
> > have a ts capability function registered for each netdev that can be
> > used by the core stack and that powers the ethtool operation as well
> > instead of the existing callback for ethtool?
> 
> Adding a check which only need to runs once in the lifetime of
> the driver to the fastpath may be a little awkward. Another option
> would be a sufficiently intelligent grep, which would understand
> which files constitute a driver. At which point grepping for 
> the ethtool op and skb_tx_timestamp would be trivial?

Many may not define the flags themselves, but defer this to
ethtool_op_get_ts_info.

A not so much intelligent, but sufficiently ugly, grep indicates
not a a massive amount of many missing entries among ethernet
drivers. But this first attempt is definitely lossy.

$ for symbol in skb_tx_timestamp get_ts_info SOF_TIMESTAMPING_TX_SOFTWARE ethtool_op_get_ts_info "(SOF_TIMESTAMPING_TX_SOFTWARE|ethtool_op_get_ts_info)"; do
    echo -n "$symbol: ";
    for i in `grep -nrIE "$symbol" drivers/net/ethernet/ | awk '{print $1}' | xargs dirname | uniq`; do echo $i; done | wc -l;
  done

skb_tx_timestamp: 69
get_ts_info: 66
SOF_TIMESTAMPING_TX_SOFTWARE: 33
ethtool_op_get_ts_info: 40
(SOF_TIMESTAMPING_TX_SOFTWARE|ethtool_op_get_ts_info): 59

This does not add up, but that's because some drivers share prefixes,
and some drivers have different paths where one open codes and the
other calls ethtool_op_get_ts_info. Marvell is a good example of both:

$ grep -nrIE '(SOF_TIMESTAMPING_TX_SOFTWARE|ethtool_op_get_ts_info)' drivers/net/ethernet
/marvell
drivers/net/ethernet/marvell/pxa168_eth.c:1367: .get_ts_info    = ethtool_op_get_ts_info,
drivers/net/ethernet/marvell/mv643xx_eth.c:1756:        .get_ts_info            = ethtool_op_get_ts_info,
drivers/net/ethernet/marvell/mvpp2/mvpp2_main.c:5266:   info->so_timestamping = SOF_TIMESTAMPING_TX_SOFTWARE |
drivers/net/ethernet/marvell/octeontx2/nic/otx2_ethtool.c:962:          return ethtool_op_get_ts_info(netdev, info);
drivers/net/ethernet/marvell/octeontx2/nic/otx2_ethtool.c:964:  info->so_timestamping = SOF_TIMESTAMPING_TX_SOFTWARE |

One more aside, no driver should have to advertise
SOF_TIMESTAMPING_SOFTWARE or SOF_TIMESTAMPING_RAW_HARDWARE. Per
Documentation/networking/timestamping.rst these are reporting flags,
not recording flags. Devices only optionall record a timestamp.