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Fri, 12 Dec 2025 19:43:19 +0000 Message-ID: <3be0dc11-8fea-4159-9f58-40594ef469f8@intel.com> Date: Fri, 12 Dec 2025 11:43:17 -0800 User-Agent: Mozilla Thunderbird Subject: Re: [Intel-wired-lan] [PATCH net v1] idpf: read lower clock bits inside the time sandwich To: Przemek Kitszel , "Loktionov, Aleksandr" , Mina Almasry CC: "Nguyen, Anthony L" , Andrew Lunn , "David S. Miller" , "netdev@vger.kernel.org" , Eric Dumazet , "linux-kernel@vger.kernel.org" , Jakub Kicinski , Paolo Abeni , Richard Cochran , "Rizzo, Luigi" , "namangulati@google.com" , "willemb@google.com" , "intel-wired-lan@lists.osuosl.org" , "Olech, Milena" , Shachar Raindel References: <20251211101931.2788437-1-almasrymina@google.com> <25a83e83-46d1-4a16-9383-6d492cb37c7a@intel.com> <684b2ce6-c8f4-4ecf-9fbc-b75137689083@intel.com> Content-Language: en-US From: Jacob Keller Autocrypt: addr=jacob.e.keller@intel.com; keydata= xjMEaFx9ShYJKwYBBAHaRw8BAQdAE+TQsi9s60VNWijGeBIKU6hsXLwMt/JY9ni1wnsVd7nN J0phY29iIEtlbGxlciA8amFjb2IuZS5rZWxsZXJAaW50ZWwuY29tPsKTBBMWCgA7FiEEIEBU qdczkFYq7EMeapZdPm8PKOgFAmhcfUoCGwMFCwkIBwICIgIGFQoJCAsCBBYCAwECHgcCF4AA CgkQapZdPm8PKOiZAAEA4UV0uM2PhFAw+tlK81gP+fgRqBVYlhmMyroXadv0lH4BAIf4jLxI UPEL4+zzp4ekaw8IyFz+mRMUBaS2l+cpoBUBzjgEaFx9ShIKKwYBBAGXVQEFAQEHQF386lYe MPZBiQHGXwjbBWS5OMBems5rgajcBMKc4W4aAwEIB8J4BBgWCgAgFiEEIEBUqdczkFYq7EMe apZdPm8PKOgFAmhcfUoCGwwACgkQapZdPm8PKOjbUQD+MsPBANqBUiNt+7w0dC73R6UcQzbg cFx4Yvms6cJjeD4BAKf193xbq7W3T7r9BdfTw6HRFYDiHXgkyoc/2Q4/T+8H In-Reply-To: <684b2ce6-c8f4-4ecf-9fbc-b75137689083@intel.com> Content-Type: multipart/signed; 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boundary="------------kTlDGRdjrsjQGsUpjKonKJ9O"; protected-headers="v1" Message-ID: <3be0dc11-8fea-4159-9f58-40594ef469f8@intel.com> Date: Fri, 12 Dec 2025 11:43:17 -0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [Intel-wired-lan] [PATCH net v1] idpf: read lower clock bits inside the time sandwich To: Przemek Kitszel , "Loktionov, Aleksandr" , Mina Almasry Cc: "Nguyen, Anthony L" , Andrew Lunn , "David S. Miller" , "netdev@vger.kernel.org" , Eric Dumazet , "linux-kernel@vger.kernel.org" , Jakub Kicinski , Paolo Abeni , Richard Cochran , "Rizzo, Luigi" , "namangulati@google.com" , "willemb@google.com" , "intel-wired-lan@lists.osuosl.org" , "Olech, Milena" , Shachar Raindel References: <20251211101931.2788437-1-almasrymina@google.com> <25a83e83-46d1-4a16-9383-6d492cb37c7a@intel.com> <684b2ce6-c8f4-4ecf-9fbc-b75137689083@intel.com> Content-Language: en-US From: Jacob Keller Autocrypt: addr=jacob.e.keller@intel.com; keydata= xjMEaFx9ShYJKwYBBAHaRw8BAQdAE+TQsi9s60VNWijGeBIKU6hsXLwMt/JY9ni1wnsVd7nN J0phY29iIEtlbGxlciA8amFjb2IuZS5rZWxsZXJAaW50ZWwuY29tPsKTBBMWCgA7FiEEIEBU qdczkFYq7EMeapZdPm8PKOgFAmhcfUoCGwMFCwkIBwICIgIGFQoJCAsCBBYCAwECHgcCF4AA CgkQapZdPm8PKOiZAAEA4UV0uM2PhFAw+tlK81gP+fgRqBVYlhmMyroXadv0lH4BAIf4jLxI UPEL4+zzp4ekaw8IyFz+mRMUBaS2l+cpoBUBzjgEaFx9ShIKKwYBBAGXVQEFAQEHQF386lYe MPZBiQHGXwjbBWS5OMBems5rgajcBMKc4W4aAwEIB8J4BBgWCgAgFiEEIEBUqdczkFYq7EMe apZdPm8PKOgFAmhcfUoCGwwACgkQapZdPm8PKOjbUQD+MsPBANqBUiNt+7w0dC73R6UcQzbg cFx4Yvms6cJjeD4BAKf193xbq7W3T7r9BdfTw6HRFYDiHXgkyoc/2Q4/T+8H In-Reply-To: <684b2ce6-c8f4-4ecf-9fbc-b75137689083@intel.com> --------------kTlDGRdjrsjQGsUpjKonKJ9O Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable 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 On Behalf= >>>> Of Mina Almasry >>>> Sent: Thursday, December 11, 2025 11:19 AM >>>> To: netdev@vger.kernel.org; linux-kernel@vger.kernel.org >>>> Cc: Mina Almasry ; Nguyen, Anthony L >>>> ; Kitszel, Przemyslaw >>>> ; Andrew Lunn ;= >>>> David S. Miller ; Eric Dumazet >>>> ; Jakub Kicinski ; Paolo Abeni= >>>> ; Richard Cochran ; >>>> Rizzo, Luigi ; namangulati@google.com; >>>> willemb@google.com; intel-wired-lan@lists.osuosl.org; Olech, Milena >>>> ; Keller, Jacob E = ; >>>> Shachar Raindel >>>> Subject: [Intel-wired-lan] [PATCH net v1] idpf: read lower clock bit= s >>>> 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@google.com >>>> Cc: namangulati@google.com >>>> Cc: willemb@google.com >>>> Cc: intel-wired-lan@lists.osuosl.org >>>> Cc: milena.olech@intel.com >>>> Cc: jacob.e.keller@intel.com >>>> >>>> Fixes: 5cb8805d2366 ("idpf: negotiate PTP capabilities and get PTP >>>> clock") >>>> Suggested-by: Shachar Raindel >>>> Signed-off-by: Mina Almasry >>>> --- >>>> 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 =3D 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_re= ad_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 tim= e, >> 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. >=20 > very nice explanation Jake, thank you >=20 > 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 ;) >=20 >> >> I think the patch is fine-as-is. >=20 > given the scope of the function, agree >=20 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. --------------kTlDGRdjrsjQGsUpjKonKJ9O-- --------------52yDz0nZ0emAotiA5680pBCZ Content-Type: application/pgp-signature; name="OpenPGP_signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="OpenPGP_signature.asc" -----BEGIN PGP SIGNATURE----- wnsEABYIACMWIQQgQFSp1zOQVirsQx5qll0+bw8o6AUCaTxwVQUDAAAAAAAKCRBqll0+bw8o6FV0 AP9v1DL+k1VOULovFAor1lQPnYP7aKq8MM1wBVoMOUYMsQD8D8OfGdOasMbOJGJh0+x7qL5TOuUG WXI76B433XEI5Q8= =Osgc -----END PGP SIGNATURE----- --------------52yDz0nZ0emAotiA5680pBCZ--