linux-kernel - Re: [RFC PATCH] net: macb: Process tx timestamp only on ptp packets

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Message-ID: <20210824140542.GA17195@hoboy.vegasvil.org>
Date:   Tue, 24 Aug 2021 07:05:42 -0700
From:   Richard Cochran <richardcochran@...il.com>
To:     Harini Katakam <harini.katakam@...inx.com>
Cc:     nicolas.ferre@...rochip.com, davem@...emloft.net,
        claudiu.beznea@...rochip.com, andrei.pistirica@...rochip.com,
        kuba@...nel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, michal.simek@...inx.com,
        harinikatakamlinux@...il.com
Subject: Re: [RFC PATCH] net: macb: Process tx timestamp only on ptp packets

On Tue, Aug 24, 2021 at 03:42:38PM +0530, Harini Katakam wrote:
> The current implementation timestamps all packets and also processes
> the BD timestamp for the same. While it is true that HWTSTAMP_TX_ON
> enables timestamps for outgoing packets, the sender of the packet
> i.e. linuxptp enables timestamp for PTP or PTP event packets. Cadence
> GEM IP has a provision to enable this in HW only for PTP packets.
> Enable this option in DMA BD settings register to decrease overhead.

NAK, because the HWTSTAMP_TX_ON means to time stamp any frame marked
by user space, not just PTP frames.

This patch does not "decrease overhead" because the code tests whether
time stamping was request per packet:

drivers/net/ethernet/cadence/macb_main.c line 1202

	if (unlikely(skb_shinfo(skb)->tx_flags &
		     SKBTX_HW_TSTAMP) &&
	    gem_ptp_do_txstamp(queue, skb, desc) == 0) {
		...
	}

Thanks,
Richard