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

["Song, Yoong Siang" <yoong.siang.song@intel.com>]

On Friday, April 14, 2023 5:50 PM, Jesper Dangaard Brouer <jbrouer@redhat.com> wrote:
>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)

Ya, 1K = BIT(10), so need to shift right 10 bits.

>
>> +#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.

It is in 64 = BIT(6) resolution, so need to shift right 6 bits.
But it start on 8th bit, so need to shift left 8 bits.
Thus, total = shift left 2 bits.

I dint put the explanation into the header file because it is too lengthy
and user can know from databook.

How do you feel on the necessary of explaining the shifting logic?
 
>
>> +#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.
>

You are right. Using FIELD_PREP() make the code clearer. 
Thanks for your suggestion. I will submit v3 for it.

>
>>   #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;
>>