linux-kernel - RE: [Intel-wired-lan] [PATCH net v1] idpf: read lower clock bits inside the time sandwich

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Message-ID: <IA3PR11MB8986D6D6FEB5B8D443E6F087E5A1A@IA3PR11MB8986.namprd11.prod.outlook.com>
Date: Thu, 11 Dec 2025 10:37:34 +0000
From: "Loktionov, Aleksandr" <aleksandr.loktionov@...el.com>
To: Mina Almasry <almasrymina@...gle.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>, "Kitszel, Przemyslaw"
	<przemyslaw.kitszel@...el.com>, Andrew Lunn <andrew+netdev@...n.ch>, "David
 S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, "Jakub
 Kicinski" <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Richard Cochran
	<richardcochran@...il.com>, "Rizzo, Luigi" <lrizzo@...gle.com>,
	"namangulati@...gle.com" <namangulati@...gle.com>, "willemb@...gle.com"
	<willemb@...gle.com>, "intel-wired-lan@...ts.osuosl.org"
	<intel-wired-lan@...ts.osuosl.org>, "Olech, Milena" <milena.olech@...el.com>,
	"Keller, Jacob E" <jacob.e.keller@...el.com>, Shachar Raindel
	<shacharr@...gle.com>
Subject: RE: [Intel-wired-lan] [PATCH net v1] idpf: read lower clock bits
 inside the time sandwich



> -----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@...osl.org> On Behalf
> Of Mina Almasry
> Sent: Thursday, December 11, 2025 11:19 AM
> To: netdev@...r.kernel.org; linux-kernel@...r.kernel.org
> Cc: Mina Almasry <almasrymina@...gle.com>; Nguyen, Anthony L
> <anthony.l.nguyen@...el.com>; Kitszel, Przemyslaw
> <przemyslaw.kitszel@...el.com>; Andrew Lunn <andrew+netdev@...n.ch>;
> David S. Miller <davem@...emloft.net>; Eric Dumazet
> <edumazet@...gle.com>; Jakub Kicinski <kuba@...nel.org>; Paolo Abeni
> <pabeni@...hat.com>; Richard Cochran <richardcochran@...il.com>;
> Rizzo, Luigi <lrizzo@...gle.com>; namangulati@...gle.com;
> willemb@...gle.com; intel-wired-lan@...ts.osuosl.org; Olech, Milena
> <milena.olech@...el.com>; Keller, Jacob E <jacob.e.keller@...el.com>;
> Shachar Raindel <shacharr@...gle.com>
> Subject: [Intel-wired-lan] [PATCH net v1] idpf: read lower clock bits
> inside the time sandwich
> 
> PCIe reads need to be done inside the time sandwich because PCIe
> writes may get buffered in the PCIe fabric and posted to the device
> after the _postts completes. Doing the PCIe read inside the time
> sandwich guarantees that the write gets flushed before the _postts
> timestamp is taken.
> 
> Cc: lrizzo@...gle.com
> Cc: namangulati@...gle.com
> Cc: willemb@...gle.com
> Cc: intel-wired-lan@...ts.osuosl.org
> Cc: milena.olech@...el.com
> Cc: jacob.e.keller@...el.com
> 
> Fixes: 5cb8805d2366 ("idpf: negotiate PTP capabilities and get PTP
> clock")
> Suggested-by: Shachar Raindel <shacharr@...gle.com>
> Signed-off-by: Mina Almasry <almasrymina@...gle.com>
> ---
>  drivers/net/ethernet/intel/idpf/idpf_ptp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/intel/idpf/idpf_ptp.c
> b/drivers/net/ethernet/intel/idpf/idpf_ptp.c
> index 3e1052d070cf..0a8b50350b86 100644
> --- a/drivers/net/ethernet/intel/idpf/idpf_ptp.c
> +++ b/drivers/net/ethernet/intel/idpf/idpf_ptp.c
> @@ -108,11 +108,11 @@ static u64
> idpf_ptp_read_src_clk_reg_direct(struct idpf_adapter *adapter,
>  	ptp_read_system_prets(sts);
> 
>  	idpf_ptp_enable_shtime(adapter);
> +	lo = readl(ptp->dev_clk_regs.dev_clk_ns_l);
The high 32 bits (hi) are still read outside the time sandwich (after ptp_read_system_postts()),
which defeats the stated purpose of ensuring PCIe write flush before timestamp capture.
/* I think he "time sandwich" is defined by the region between ptp_read_system_prets(sts) and ptp_read_system_postts(sts)  */ Isn't it?


> 
>  	/* Read the system timestamp post PHC read */
>  	ptp_read_system_postts(sts);
> 
> -	lo = readl(ptp->dev_clk_regs.dev_clk_ns_l);
>  	hi = readl(ptp->dev_clk_regs.dev_clk_ns_h);
> 
>  	spin_unlock(&ptp->read_dev_clk_lock);
> 
> base-commit: 885bebac9909994050bbbeed0829c727e42bd1b7
> --
> 2.52.0.223.gf5cc29aaa4-goog