[xdp-hints] Re: [PATCH net v2 1/1] igc: read before write to SRRCTL register

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

On 14/04/2023 04.09, 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.
> 
> Command on DUT:
>    sudo ./xdp_hw_metadata <interface name>
> 
> Command on Link Partner:
>    echo -n skb | nc -u -q1 <destination IPv4 addr> 9092
> 
> Result before this patch:
>    skb hwtstamp is not found!
> 
> Result after this patch:
>    found skb hwtstamp = 1677800973.642836757
> 
> Optionally, read PHC to confirm the values obtained are almost the same:
> Command:
>    sudo ./testptp -d /dev/ptp0 -g
> Result:
>    clock time: 1677800973.913598978 or Fri Mar  3 07:49:33 2023
> 
> 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>

> v2 changelog:
>   - Fix indention
> ---
>   drivers/net/ethernet/intel/igc/igc_base.h | 7 +++++--
>   drivers/net/ethernet/intel/igc/igc_main.c | 5 ++++-
>   2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/igc/igc_base.h b/drivers/net/ethernet/intel/igc/igc_base.h
> index 7a992befca24..b95007d51d13 100644
> --- a/drivers/net/ethernet/intel/igc/igc_base.h
> +++ b/drivers/net/ethernet/intel/igc/igc_base.h
> @@ -87,8 +87,11 @@ union igc_adv_rx_desc {
>   #define IGC_RXDCTL_SWFLUSH		0x04000000 /* Receive Software Flush */
>   
>   /* SRRCTL bit definitions */

I have checked Foxville manual for SRRCTL (Split and Replication Receive
Control) register and below GENMASKs looks correct.

> -#define IGC_SRRCTL_BSIZEPKT_SHIFT		10 /* Shift _right_ */
> -#define IGC_SRRCTL_BSIZEHDRSIZE_SHIFT		2  /* Shift _left_ */
> +#define IGC_SRRCTL_BSIZEPKT_MASK	GENMASK(6, 0)
> +#define IGC_SRRCTL_BSIZEPKT_SHIFT	10 /* Shift _right_ */

Shift due to 1 KB resolution of BSIZEPKT (manual field BSIZEPACKET)

> +#define IGC_SRRCTL_BSIZEHDRSIZE_MASK	GENMASK(13, 8)
> +#define IGC_SRRCTL_BSIZEHDRSIZE_SHIFT	2  /* Shift _left_ */

This shift is suspicious, but as you inherited it I guess it works.
I did the math, and it happens to work, knowing (from manual) value is
in 64 bytes resolution.

> +#define IGC_SRRCTL_DESCTYPE_MASK	GENMASK(27, 25)
>   #define IGC_SRRCTL_DESCTYPE_ADV_ONEBUF	0x02000000

Given you have started using GENMASK(), then I would have updated 
IGC_SRRCTL_DESCTYPE_ADV_ONEBUF to be expressed like:

  #define IGC_SRRCTL_DESCTYPE_ADV_ONEBUF 
FIELD_PREP(IGC_SRRCTL_DESCTYPE_MASK, 0x1)

Making it easier to see code is selecting:
  001b = Advanced descriptor one buffer.

And not (as I first though):
  010b = Advanced descriptor header splitting.


>   #endif /* _IGC_BASE_H */
> diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
> index 25fc6c65209b..88fac08d8a14 100644
> --- a/drivers/net/ethernet/intel/igc/igc_main.c
> +++ b/drivers/net/ethernet/intel/igc/igc_main.c
> @@ -641,7 +641,10 @@ static void igc_configure_rx_ring(struct igc_adapter *adapter,
>   	else
>   		buf_size = IGC_RXBUFFER_2048;
>   
> -	srrctl = IGC_RX_HDR_LEN << IGC_SRRCTL_BSIZEHDRSIZE_SHIFT;
> +	srrctl = rd32(IGC_SRRCTL(reg_idx));
> +	srrctl &= ~(IGC_SRRCTL_BSIZEPKT_MASK | IGC_SRRCTL_BSIZEHDRSIZE_MASK |
> +		    IGC_SRRCTL_DESCTYPE_MASK);
> +	srrctl |= IGC_RX_HDR_LEN << IGC_SRRCTL_BSIZEHDRSIZE_SHIFT;
>   	srrctl |= buf_size >> IGC_SRRCTL_BSIZEPKT_SHIFT;
>   	srrctl |= IGC_SRRCTL_DESCTYPE_ADV_ONEBUF;
>