Subject: Re: [PATCH net-next v1] net: stmmac: Caclucate clock domain crossing error only once

From: Thomas Gleixner <[email-masked]>
Date: Fri, 19 Nov 2021 12:50:21 +0100

Kurt,

On Fri, Nov 19 2021 at 09:10, Kurt Kanzenbach wrote:

> The clock domain crossing error (CDC) is calculated at every fetch of Tx or Rx
> timestamps. It includes a division. Especially on arm32 based systems it is
> expensive. It also saves the two conditionals.

This does not make sense. What you want to say here is:

  It also requires two conditionals in the hotpath.

> Therefore, move the calculation to the PTP initialization code and just use the
> cached value in the timestamp retrieval functions.

Maybe:

  Add a compensation value cache to struct plat_stmmacenet_data and
  subtract it unconditionally in the RX/TX functions which spares the
  conditionals.

  The value is initialized to 0 and if supported calculated in the PTP
  initialization code.

or something to that effect.

> +	/* Calculate the clock domain crossing (CDC) error if necessary */
> +	priv->plat->cdc_error_adj = 0;
> +	if (priv->plat->has_gmac4 && priv->plat->clk_ptp_rate)
> +		priv->plat->cdc_error_adj = (2 * NSEC_PER_SEC) /
> +			priv->plat->clk_ptp_rate;

Nit. Just let stick it out. We lifted the 80 char limitation some time ago.

Thanks,

        tglx