Re: [PATCH] i40e: fix the wrong PTP frequency calculation

[Yajun Deng <yajun.deng@linux.dev>]

On 2023/6/28 04:20, Jacob Keller wrote:
>
> On 6/26/2023 7:26 PM, Yajun Deng wrote:
>> The new adjustment should be based on the base frequency, not the
>> I40E_PTP_40GB_INCVAL in i40e_ptp_adjfine().
>>
>> This issue was introduced in commit 3626a690b717 ("i40e: use
>> mul_u64_u64_div_u64 for PTP frequency calculation"), and was fixed in
>> commit 1060707e3809 ("ptp: introduce helpers to adjust by scaled
>> parts per million"). However the latter is a new feature and hasn't been
>> backported to the stable releases.
>>
>> This issue affects both v6.0 and v6.1 versions, and the v6.1 version is
>> an LTS version.
>>
>> Fixes: 3626a690b717 ("i40e: use mul_u64_u64_div_u64 for PTP frequency calculation")
>> Cc: <stable@vger.kernel.org> # 6.1
>> Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
>> ---
>>   drivers/net/ethernet/intel/i40e/i40e_ptp.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/intel/i40e/i40e_ptp.c b/drivers/net/ethernet/intel/i40e/i40e_ptp.c
>> index ffea0c9c82f1..97a9efe7b713 100644
>> --- a/drivers/net/ethernet/intel/i40e/i40e_ptp.c
>> +++ b/drivers/net/ethernet/intel/i40e/i40e_ptp.c
>> @@ -361,9 +361,9 @@ static int i40e_ptp_adjfine(struct ptp_clock_info *ptp, long scaled_ppm)
>>   				   1000000ULL << 16);
>>   
>>   	if (neg_adj)
>> -		adj = I40E_PTP_40GB_INCVAL - diff;
>> +		adj = freq - diff;
>>   	else
>> -		adj = I40E_PTP_40GB_INCVAL + diff;
>> +		adj = freq + diff;
>>   
>>   	wr32(hw, I40E_PRTTSYN_INC_L, adj & 0xFFFFFFFF);
>>   	wr32(hw, I40E_PRTTSYN_INC_H, adj >> 32);
> This straight forward fix makes sense. However, it wasn't obvious to me
> without context why the 3626a690b717 ("i40e: use mul_u64_u64_div_u64 for
> PTP frequency calculation") was where the fault got introduced. Thus,
> here is that context for anyone else who failed to spot it just looking
> at shrunk patch diffs.
>
> --> code before that commit <---
>> /**
>>   * i40e_ptp_adjfreq - Adjust the PHC frequency
>>   * @ptp: The PTP clock structure
>>   * @ppb: Parts per billion adjustment from the base
>>   *
>>   * Adjust the frequency of the PHC by the indicated parts per billion from the
>>   * base frequency.
>>   **/
>> static int i40e_ptp_adjfreq(struct ptp_clock_info *ptp, s32 ppb)
>> {
>>          struct i40e_pf *pf = container_of(ptp, struct i40e_pf, ptp_caps);
>>          struct i40e_hw *hw = &pf->hw;
>>          u64 adj, freq, diff;
>>          int neg_adj = 0;
>>
>>          if (ppb < 0) {
>>                  neg_adj = 1;
>>                  ppb = -ppb;
>>          }
>>
>>          freq = I40E_PTP_40GB_INCVAL;
> frequency is left just as base I40E_PTP_40GB_INCVAL.
>
>>          freq *= ppb;
>>          diff = div_u64(freq, 1000000000ULL);
>>
>>          if (neg_adj)
>>                  adj = I40E_PTP_40GB_INCVAL - diff;
>>          else
>>                  adj = I40E_PTP_40GB_INCVAL + diff;
>>
> So the base here can't be freq since we modify freq above using *=, but
> using I40E_PTP_40GB_INCVAL is consistent.
>
>>          /* At some link speeds, the base incval is so large that directly
>>           * multiplying by ppb would result in arithmetic overflow even when
>>           * using a u64. Avoid this by instead calculating the new incval
>>           * always in terms of the 40GbE clock rate and then multiplying by the
>>           * link speed factor afterwards. This does result in slightly lower
>>           * precision at lower link speeds, but it is fairly minor.
>>           */
>>          smp_mb(); /* Force any pending update before accessing. */
>>          adj *= READ_ONCE(pf->ptp_adj_mult);
>>
> Finally, the multiply is applied last. This affects the combined base +
> difference, and is done in order to avoid overflowing the *= used in the
> original implementation.
>
>>          wr32(hw, I40E_PRTTSYN_INC_L, adj & 0xFFFFFFFF);
>>          wr32(hw, I40E_PRTTSYN_INC_H, adj >> 32);
>>
>>          return 0;
>> }
>
> ---> code after that commit <---
>> /**
>>   * i40e_ptp_adjfreq - Adjust the PHC frequency
>>   * @ptp: The PTP clock structure
>>   * @ppb: Parts per billion adjustment from the base
>>   *
>>   * Adjust the frequency of the PHC by the indicated parts per billion from the
>>   * base frequency.
>>   **/
>> static int i40e_ptp_adjfreq(struct ptp_clock_info *ptp, s32 ppb)
>> {
>>          struct i40e_pf *pf = container_of(ptp, struct i40e_pf, ptp_caps);
>>          struct i40e_hw *hw = &pf->hw;
>>          u64 adj, freq, diff;
>>          int neg_adj = 0;
>>
>>          if (ppb < 0) {
>>                  neg_adj = 1;
>>                  ppb = -ppb;
>>          }
>>
>>          smp_mb(); /* Force any pending update before accessing. */
>>          freq = I40E_PTP_40GB_INCVAL * READ_ONCE(pf->ptp_adj_mult);
>>          diff = mul_u64_u64_div_u64(freq, (u64)ppb,
>>                                     1000000000ULL);
>>
> Here, we assign freq to be the I40E_PTP_40GB_INCVAL times the
> ptp_adj_mult value, and then we don't modify it, instead using
> mul_u64_u64_div_u64.
>
>>          if (neg_adj)
>>                  adj = I40E_PTP_40GB_INCVAL - diff;
>>          else
>>                  adj = I40E_PTP_40GB_INCVAL + diff;
>>
> But then the diff is applied on the wrong value, and no multiplication
> is done afterwards.
>
>>          wr32(hw, I40E_PRTTSYN_INC_L, adj & 0xFFFFFFFF);
>>          wr32(hw, I40E_PRTTSYN_INC_H, adj >> 32);
>>
>>          return 0;
>> }
> ---> current version with adjust_by_scaled_ppm <---
>> /**
>>   * i40e_ptp_adjfine - Adjust the PHC frequency
>>   * @ptp: The PTP clock structure
>>   * @scaled_ppm: Scaled parts per million adjustment from base
>>   *
>>   * Adjust the frequency of the PHC by the indicated delta from the base
>>   * frequency.
>>   *
>>   * Scaled parts per million is ppm with a 16 bit binary fractional field.
>>   **/
>> static int i40e_ptp_adjfine(struct ptp_clock_info *ptp, long scaled_ppm)
>> {
>>          struct i40e_pf *pf = container_of(ptp, struct i40e_pf, ptp_caps);
>>          struct i40e_hw *hw = &pf->hw;
>>          u64 adj, base_adj;
>>
>>          smp_mb(); /* Force any pending update before accessing. */
>>          base_adj = I40E_PTP_40GB_INCVAL * READ_ONCE(pf->ptp_adj_mult);
>>
>>          adj = adjust_by_scaled_ppm(base_adj, scaled_ppm);
>>
> Using adjust_by_scaled_ppm correctly performs the calculation and uses
> the base adjustment, so there's no error here.
>
>>          wr32(hw, I40E_PRTTSYN_INC_L, adj & 0xFFFFFFFF);
>>          wr32(hw, I40E_PRTTSYN_INC_H, adj >> 32);
>>
>>          return 0;
>> }
>
> Thanks for finding and fixing this mistake. I think its the simplest fix
> to get into the stable kernel that are broken, since taking the
> adjust_by_scaled_ppm version would require additional patches.
>
> Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
>
Kindly ping...