linux-kernel - Re: [PATCH v2] net: stmmac: correct MAC propagation delay

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Date:   Wed, 26 Jul 2023 13:57:28 -0700
From:   Richard Cochran <richardcochran@...il.com>
To:     Johannes Zink <j.zink@...gutronix.de>
Cc:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Russell King <linux@...linux.org.uk>,
        patchwork-jzi@...gutronix.de, netdev@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kernel@...gutronix.de, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v2] net: stmmac: correct MAC propagation delay

On Mon, Jul 24, 2023 at 12:01:31PM +0200, Johannes Zink wrote:

Earlier versions of the IP core return zero from these...

> +#define	PTP_TS_INGR_LAT	0x68	/* MAC internal Ingress Latency */
> +#define	PTP_TS_EGR_LAT	0x6c	/* MAC internal Egress Latency */

and so...

> +static void correct_latency(struct stmmac_priv *priv)
> +{
> +	void __iomem *ioaddr = priv->ptpaddr;
> +	u32 reg_tsic, reg_tsicsns;
> +	u32 reg_tsec, reg_tsecsns;
> +	u64 scaled_ns;
> +	u32 val;
> +
> +	/* MAC-internal ingress latency */
> +	scaled_ns = readl(ioaddr + PTP_TS_INGR_LAT);
> +
> +	/* See section 11.7.2.5.3.1 "Ingress Correction" on page 4001 of
> +	 * i.MX8MP Applications Processor Reference Manual Rev. 1, 06/2021
> +	 */
> +	val = readl(ioaddr + PTP_TCR);
> +	if (val & PTP_TCR_TSCTRLSSR)
> +		/* nanoseconds field is in decimal format with granularity of 1ns/bit */
> +		scaled_ns = ((u64)NSEC_PER_SEC << 16) - scaled_ns;
> +	else
> +		/* nanoseconds field is in binary format with granularity of ~0.466ns/bit */
> +		scaled_ns = ((1ULL << 31) << 16) -
> +			DIV_U64_ROUND_CLOSEST(scaled_ns * PSEC_PER_NSEC, 466U);
> +
> +	reg_tsic = scaled_ns >> 16;
> +	reg_tsicsns = scaled_ns & 0xff00;
> +
> +	/* set bit 31 for 2's compliment */
> +	reg_tsic |= BIT(31);
> +
> +	writel(reg_tsic, ioaddr + PTP_TS_INGR_CORR_NS);

here reg_tsic = 0x80000000 for a correction of -2.15 seconds!

@Jakub  Can you please revert this patch?

Thanks,
Richard