[xdp-hints] Re: [PATCH net v3 1/1] igc: read before write to SRRCTL register

[Florian Bezdeka <florian.bezdeka@siemens.com>]

On Mon, 2023-04-17 at 16:24 +0200, Jesper Dangaard Brouer wrote:
> On 14/04/2023 22.05, Jesper Dangaard Brouer wrote:
> >  
> > On 14/04/2023 17.49, Song Yoong Siang wrote:
> > > igc_configure_rx_ring() function will be called as part of XDP program
> > > setup. If Rx hardware timestamp is enabled prio to XDP program setup,
> > > this timestamp enablement will be overwritten when buffer size is
> > > written into SRRCTL register.
> > > 
> > > Thus, this commit read the register value before write to SRRCTL
> > > register. This commit is tested by using xdp_hw_metadata bpf selftest
> > > tool. The tool enables Rx hardware timestamp and then attach XDP program
> > > to igc driver. It will display hardware timestamp of UDP packet with
> > > port number 9092. Below are detail of test steps and results.
> > > 
> [...]
> > > 
> > > Fixes: fc9df2a0b520 ("igc: Enable RX via AF_XDP zero-copy")
> > > Cc: <stable@vger.kernel.org> # 5.14+
> > > Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
> > > Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
> > > Reviewed-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > > ---
> > 
> > LGTM, thank for the adjustments :-)
> > 
> > Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > 
> 
> Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
> 
> I can confirm that this patch fix the issue I experienced with igc.
> 
> This patch clearly fixes a bug in igc when writing the SRRCTL register.
> (as bit 30 in register is "Timestamp Received Packet" which got cleared 
> before).
> 
> Florian might have found another bug around RX timestamps, but this
> patch should be safe and sane to apply as is.

After a closer look I'm quite sure now that this patch should fix my
issue as well. The register will be overwritten when setting up a
XSK_POOL as well:

igc_bpf
  igc_xdp_setup_pool
    igc_enable_rx_ring
      igc_configure_rx_ring
        wr32(IGC_SRRCTL)

I already removed the BPF loading (which is the use case that the patch
description mentions) from my setup to limit the search scope. If you
like you could extend the patch description, but I'm fine with it.

Thanks a lot for all the support / ideas! Highly appreciated!

Florian

> 
> > > v2 -> v3: Refactor SRRCTL definitions to more human readable definitions
> > > v1 -> v2: Fix indention
> > > ---
> > >   drivers/net/ethernet/intel/igc/igc_base.h | 11 ++++++++---
> > >   drivers/net/ethernet/intel/igc/igc_main.c |  7 +++++--
> > >   2 files changed, 13 insertions(+), 5 deletions(-)
>