Subject: Re: [PATCH 4/4] net: dsa: mv88e6xxx: add PTP support for MV88E6250 family

From: Richard Cochran <[email-masked]>
Date: Tue, 30 Jul 2019 09:00:32 -0700

On Tue, Jul 30, 2019 at 12:04:29PM +0200, Hubert Feurstein wrote:
> diff --git a/drivers/net/dsa/mv88e6xxx/ptp.c b/drivers/net/dsa/mv88e6xxx/ptp.c
> index 768d256f7c9f..51cdf4712517 100644
> --- a/drivers/net/dsa/mv88e6xxx/ptp.c
> +++ b/drivers/net/dsa/mv88e6xxx/ptp.c
> @@ -15,11 +15,38 @@
>  #include "hwtstamp.h"
>  #include "ptp.h"
>  
> -/* Raw timestamps are in units of 8-ns clock periods. */
> -#define CC_SHIFT	28
> -#define CC_MULT		(8 << CC_SHIFT)
> -#define CC_MULT_NUM	(1 << 9)
> -#define CC_MULT_DEM	15625ULL
> +/* The adjfine API clamps ppb between [-32,768,000, 32,768,000], and

That is not true.

> + * therefore scaled_ppm between [-2,147,483,648, 2,147,483,647].
> + * Set the maximum supported ppb to a round value smaller than the maximum.
> + *
> + * Percentually speaking, this is a +/- 0.032x adjustment of the
> + * free-running counter (0.968x to 1.032x).
> + */
> +#define MV88E6XXX_MAX_ADJ_PPB	32000000

I had set an arbitrary limit of 1000 ppm.  I can't really see any
point in raising the limit.

> +/* Family MV88E6250:
> + * Raw timestamps are in units of 10-ns clock periods.
> + *
> + * clkadj = scaled_ppm * 10*2^28 / (10^6 * 2^16)
> + * simplifies to
> + * clkadj = scaled_ppm * 2^7 / 5^5
> + */
> +#define MV88E6250_CC_SHIFT	28
> +#define MV88E6250_CC_MULT	(10 << MV88E6250_CC_SHIFT)
> +#define MV88E6250_CC_MULT_NUM	(1 << 7)
> +#define MV88E6250_CC_MULT_DEM	3125ULL
> +
> +/* Other families:
> + * Raw timestamps are in units of 8-ns clock periods.
> + *
> + * clkadj = scaled_ppm * 8*2^28 / (10^6 * 2^16)
> + * simplifies to
> + * clkadj = scaled_ppm * 2^9 / 5^6
> + */
> +#define MV88E6XXX_CC_SHIFT	28
> +#define MV88E6XXX_CC_MULT	(8 << MV88E6XXX_CC_SHIFT)
> +#define MV88E6XXX_CC_MULT_NUM	(1 << 9)
> +#define MV88E6XXX_CC_MULT_DEM	15625ULL
>  
>  #define TAI_EVENT_WORK_INTERVAL msecs_to_jiffies(100)
>  
> @@ -179,24 +206,14 @@ static void mv88e6352_tai_event_work(struct work_struct *ugly)
>  static int mv88e6xxx_ptp_adjfine(struct ptp_clock_info *ptp, long scaled_ppm)
>  {
>  	struct mv88e6xxx_chip *chip = ptp_to_chip(ptp);
> -	int neg_adj = 0;
> -	u32 diff, mult;
> -	u64 adj;
> +	s64 adj;
>  
> -	if (scaled_ppm < 0) {
> -		neg_adj = 1;
> -		scaled_ppm = -scaled_ppm;
> -	}

Please don't re-write this logic.  It is written like that for a reason.

> -	mult = CC_MULT;
> -	adj = CC_MULT_NUM;
> -	adj *= scaled_ppm;
> -	diff = div_u64(adj, CC_MULT_DEM);

Just substitute CC_MULT* with your new chip->ptp_cc_mult*
and leave the rest alone.

Thanks,
Richard