On 12/11/2025 11:57 PM, Przemek Kitszel wrote: > On 12/11/25 23:06, Jacob Keller wrote: >> >> >> On 12/11/2025 2:37 AM, Loktionov, Aleksandr wrote: >>> >>> >>>> -----Original Message----- >>>> From: Intel-wired-lan <[email protected]> On Behalf >>>> Of Mina Almasry >>>> Sent: Thursday, December 11, 2025 11:19 AM >>>> To: [email protected]; [email protected] >>>> Cc: Mina Almasry <[email protected]>; Nguyen, Anthony L >>>> <[email protected]>; Kitszel, Przemyslaw >>>> <[email protected]>; Andrew Lunn <[email protected]>; >>>> David S. Miller <[email protected]>; Eric Dumazet >>>> <[email protected]>; Jakub Kicinski <[email protected]>; Paolo Abeni >>>> <[email protected]>; Richard Cochran <[email protected]>; >>>> Rizzo, Luigi <[email protected]>; [email protected]; >>>> [email protected]; [email protected]; Olech, Milena >>>> <[email protected]>; Keller, Jacob E <[email protected]>; >>>> Shachar Raindel <[email protected]> >>>> 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: [email protected] >>>> Cc: [email protected] >>>> Cc: [email protected] >>>> Cc: [email protected] >>>> Cc: [email protected] >>>> Cc: [email protected] >>>> >>>> Fixes: 5cb8805d2366 ("idpf: negotiate PTP capabilities and get PTP >>>> clock") >>>> Suggested-by: Shachar Raindel <[email protected]> >>>> Signed-off-by: Mina Almasry <[email protected]> >>>> --- >>>> 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? >>> >>> >> >> Any read will cause writes to flush, so we don't need to move both >> registers. >> >> The point here is that we write to the shadow register to snapshot time, >> and it won't guarantee to be flushed to the device until a read. By >> moving a single read in side the time sandwhich, we ensure that its >> actually complete before the time snapshot is taken. We don't need to >> wait for both registers because of the snapshot behavior. > > very nice explanation Jake, thank you > > I don't know if it should be considered "basic common knowledge", or > warrants an entry in commit message/code comment > For sure we don't want anyone not knowing that to touch the code, so > barrier to entry is also a good thing ;) > >> >> I think the patch is fine-as-is. > > given the scope of the function, agree > Reading both registers would take longer, and would delay post timestamp, which would lower the accuracy of the clock comparison mechanisms that use the pre+post timestamps. We *must* read one of the values because we need to ensure the PHC timestamp is snapshot between pre+post, but we should do as little work as necessary, so only reading the low register is the most optimal.
This could be put into the commit message, but at least to me as a domain expert the original commit message was sufficient, so I'm not sure that I'm a good judge for what is necessary for others to understand the logic.
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