Re: [PATCH net-next v2 08/11] net: ptp: add helper for one-step P2P clocks

[Vladimir Oltean <olteanv@gmail.com>]

On Thu, Nov 12, 2020 at 04:35:34PM +0100, Christian Eggers wrote:
> This function subtracts the ingress hardware time stamp from the
> correction field of a PTP header and updates the UDP checksum (if UDP is
> used as transport. It is needed for hardware capable of one-step P2P

Please close the parenthesis.

> that does not already modify the correction field of Pdelay_Req event
> messages on ingress.
> 
> Signed-off-by: Christian Eggers <ceggers@arri.de>

Please add more verbiage here, giving the reader as much context as
possible. You are establishing a de-facto approach for one-step peer
delay timestamping for hardware like yours, you need to explain why it
is done this way, for people to understand just by looking at git blame.

> ---
>  include/linux/ptp_classify.h | 97 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 97 insertions(+)
> 
> diff --git a/include/linux/ptp_classify.h b/include/linux/ptp_classify.h
> index 56b2d7d66177..f27b512e1abd 100644
> --- a/include/linux/ptp_classify.h
> +++ b/include/linux/ptp_classify.h
> @@ -10,8 +10,12 @@
>  #ifndef _PTP_CLASSIFY_H_
>  #define _PTP_CLASSIFY_H_
>  
> +#include <asm/unaligned.h>
>  #include <linux/ip.h>
> +#include <linux/ktime.h>
>  #include <linux/skbuff.h>
> +#include <linux/udp.h>
> +#include <net/checksum.h>
>  
>  #define PTP_CLASS_NONE  0x00 /* not a PTP event message */
>  #define PTP_CLASS_V1    0x01 /* protocol version 1 */
> @@ -118,6 +122,91 @@ static inline u8 ptp_get_msgtype(const struct ptp_header *hdr,
>  	return msgtype;
>  }
>  
> +/**
> + * ptp_check_diff8 - Computes new checksum (when altering a 64-bit field)
> + * @old: old field value
> + * @new: new field value
> + * @oldsum: previous checksum
> + *
> + * This function can be used to calculate a new checksum when only a single
> + * field is changed. Similar as ip_vs_check_diff*() in ip_vs.h.
> + *
> + * Return: Updated checksum
> + */
> +static inline __wsum ptp_check_diff8(__be64 old, __be64 new, __wsum oldsum)
> +{
> +	__be64 diff[2] = { ~old, new };
> +
> +	return csum_partial(diff, sizeof(diff), oldsum);
> +}
> +
> +/**
> + * ptp_onestep_p2p_move_t2_to_correction - Update PTP header's correction field
> + * @skb: packet buffer
> + * @type: type of the packet (see ptp_classify_raw())
> + * @hdr: ptp header
> + * @t2: ingress hardware time stamp
> + *
> + * This function subtracts the ingress hardware time stamp from the correction
> + * field of a PTP header and updates the UDP checksum (if UDP is used as
> + * transport). It is needed for hardware capable of one-step P2P that does not
> + * already modify the correction field of Pdelay_Req event messages on ingress.
> + */
> +static inline
> +void ptp_onestep_p2p_move_t2_to_correction(struct sk_buff *skb,
> +					   unsigned int type,
> +					   struct ptp_header *hdr,
> +					   ktime_t t2)

The way this function is coded up right now, there's no reason to call it:
- onestep
- p2p
- move_t2_to_correction
As it is, it would be better named ptp_header_update_correction.

> +{
> +	u8 *ptr = skb_mac_header(skb);
> +	struct udphdr *uhdr = NULL;
> +	s64 ns = ktime_to_ns(t2);
> +	__be64 correction_old;
> +	s64 correction;
> +
> +	/* previous correction value is required for checksum update. */
> +	memcpy(&correction_old,  &hdr->correction, sizeof(correction_old));
> +	correction = (s64)be64_to_cpu(correction_old);
> +
> +	/* PTP correction field consists of 32 bit nanoseconds and 16 bit

48 bit nanoseconds

> +	 * fractional nanoseconds.  Avoid shifting negative numbers.
> +	 */
> +	if (ns >= 0)
> +		correction -= ns << 16;
> +	else
> +		correction += -ns << 16;

Again? Why? There's nothing wrong with left-shifting negative numbers,
two's complement works the same (note that right-shifting is a different
story, but that's not the case here).

> +
> +	/* write new correction value */
> +	put_unaligned_be64((u64)correction, &hdr->correction);
> +
> +	/* locate udp header */
> +	if (type & PTP_CLASS_VLAN)
> +		ptr += VLAN_HLEN;

Can't you just go back from the struct ptp_header pointer and avoid this
check?

> +	ptr += ETH_HLEN;
> +
> +	switch (type & PTP_CLASS_PMASK) {
> +	case PTP_CLASS_IPV4:
> +		ptr += ((struct iphdr *)ptr)->ihl << 2;
> +		uhdr = (struct udphdr *)ptr;
> +		break;
> +	case PTP_CLASS_IPV6:
> +		ptr += IP6_HLEN;
> +		uhdr = (struct udphdr *)ptr;
> +		break;
> +	}
> +
> +	if (!uhdr)
> +		return;
> +
> +	/* update checksum */
> +	uhdr->check = csum_fold(ptp_check_diff8(correction_old,
> +						hdr->correction,
> +						~csum_unfold(uhdr->check)));
> +	if (!uhdr->check)
> +		uhdr->check = CSUM_MANGLED_0;
> +}
> +
>