Subject: Re: [PATCH net] net/mlx5: Dynamic cyclecounter shift calculation for PTP free running clock

From: Rahul Rameshbabu <[email-masked]>
Date: Wed, 23 Aug 2023 14:46:20 -0700

On Wed, 23 Aug, 2023 12:54:21 -0700 Jacob Keller <[email-masked]> wrote:
> On 8/21/2023 4:05 PM, Rahul Rameshbabu wrote:
>> Use a dynamic calculation to determine the shift value for the internal
>> timer cyclecounter that will lead to the highest precision frequency
>> adjustments. Previously used a constant for the shift value assuming all
>> devices supported by the driver had a nominal frequency of 1GHz. However,
>> there are devices that operate at different frequencies. The previous shift
>> value constant would break the PHC functionality for those devices.
>> 
>> Reported-by: Vadim Fedorenko <[email-masked]>
>> Closes: https://lore.kernel.org/netdev/[email-masked]/
>> Fixes: 6a4010927562 ("net/mlx5: Update cyclecounter shift value to improve ptp free running mode precision")
>> Signed-off-by: Rahul Rameshbabu <[email-masked]>
>> Tested-by: Vadim Fedorenko <[email-masked]>
>> ---
>> 
>
> Reviewed-by: Jacob Keller <[email-masked]>
>
>> Notes:
>>     Devices tested on:
>>     
>>       * ConnectX 4
>>       * ConnectX 4-Lx
>>       * ConnectX 5
>>       * ConnectX 6
>>       * ConnectX 6-Dx
>>       * ConnectX 7
>> 
>>  .../ethernet/mellanox/mlx5/core/lib/clock.c   | 32 ++++++++++++++++---
>>  1 file changed, 27 insertions(+), 5 deletions(-)
>> 
>> diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.c b/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.c
>> index 377372f0578a..aa29f09e8356 100644
>> --- a/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.c
>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.c
>> @@ -32,16 +32,13 @@
>>  
>>  #include <linux/clocksource.h>
>>  #include <linux/highmem.h>
>> +#include <linux/log2.h>
>>  #include <linux/ptp_clock_kernel.h>
>>  #include <rdma/mlx5-abi.h>
>>  #include "lib/eq.h"
>>  #include "en.h"
>>  #include "clock.h"
>>  
>> -enum {
>> -	MLX5_CYCLES_SHIFT	= 31
>> -};
>> -
>>  enum {
>>  	MLX5_PIN_MODE_IN		= 0x0,
>>  	MLX5_PIN_MODE_OUT		= 0x1,
>> @@ -93,6 +90,31 @@ static bool mlx5_modify_mtutc_allowed(struct mlx5_core_dev *mdev)
>>  	return MLX5_CAP_MCAM_FEATURE(mdev, ptpcyc2realtime_modify);
>>  }
>>  
>> +static u32 mlx5_ptp_shift_constant(u32 dev_freq_khz)
>> +{
>> +	/* Optimal shift constant leads to corrections above just 1 scaled ppm.
>> +	 *
>> +	 * Two sets of equations are needed to derive the optimal shift
>> +	 * constant for the cyclecounter.
>> +	 *
>> +	 *    dev_freq_khz * 1000 / 2^shift_constant = 1 scaled_ppm
>> +	 *    ppb = scaled_ppm * 1000 / 2^16
>> +	 *
>> +	 * Using the two equations together
>> +	 *
>> +	 *    dev_freq_khz * 1000 / 1 scaled_ppm = 2^shift_constant
>> +	 *    dev_freq_khz * 2^16 / 1 ppb = 2^shift_constant
>> +	 *    dev_freq_khz = 2^(shift_constant - 16)
>> +	 *
>> +	 * then yields
>> +	 *
>> +	 *    shift_constant = ilog2(dev_freq_khz) + 16
>> +	 */
>> +
>
> I appreciate the derivation here. It helps understand the calculation
> here, and makes it clear why this is the best constant. Deriving it in
> terms of the frequency is useful since it makes supporting other
> frequencies much simpler in the future if thats ever necessary for the
> device family, rather than just adding a table of known frequencies. Nice!
>
>> +	return min(ilog2(dev_freq_khz) + 16,
>> +		   ilog2((U32_MAX / NSEC_PER_MSEC) * dev_freq_khz));
>> +}
>> +
>>  static s32 mlx5_ptp_getmaxphase(struct ptp_clock_info *ptp)
>>  {
>>  	struct mlx5_clock *clock = container_of(ptp, struct mlx5_clock, ptp_info);
>> @@ -909,7 +931,7 @@ static void mlx5_timecounter_init(struct mlx5_core_dev *mdev)
>>  
>>  	dev_freq = MLX5_CAP_GEN(mdev, device_frequency_khz);
>>  	timer->cycles.read = read_internal_timer;
>> -	timer->cycles.shift = MLX5_CYCLES_SHIFT;
>> +	timer->cycles.shift = mlx5_ptp_shift_constant(dev_freq);
>>  	timer->cycles.mult = clocksource_khz2mult(dev_freq,
>>  						  timer->cycles.shift);
>
> And you already derive the multiplier in terms of the frequency and
> shift, so the change in shift won't break the multiplier. Good.
>
>>  	timer->nominal_c_mult = timer->cycles.mult;
>
>
> Not really an issue of this patch, but a few drivers use a nominal
> multiplier in calculations with timecounter and cycle counter, I wonder
> if this could be baked into the cyclecounter code in the future...

This ran through my mind as I was making this patch. As you mentioned,
the logic used here is not specific to mlx5. Rather, it's a general
calculator for the shift value given a frequency. I wanted to look at
all the use cases of cyclecounter before providing a general API for
this. I will likely follow up with you if I have concerns with regards
to the generalization for the cyclecounter API and hopefully can share
an RFC.

Thanks,

Rahul Rameshbabu

>
> At any rate, this fix looks good to me.