From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nicholas Piggin Subject: Re: [RFC 0/3] New idle device-tree format and support for versioned stop state Date: Tue, 27 Aug 2019 19:31:02 +1000 Message-ID: <1566894003.kb57vy8ri2.astroid@bobo.none> References: <20190823070940.43220-1-huntbag@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <20190823070940.43220-1-huntbag@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org To: devicetree@vger.kernel.org, Abhishek Goel , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, paulus@samba.org Cc: daniel.lezcano@linaro.org, ego@linux.vnet.ibm.com, mikey@neuling.org, mpe@ellerman.id.au, rjw@rjwysocki.net, svaidy@linux.ibm.com List-Id: devicetree@vger.kernel.org Abhishek Goel's on August 23, 2019 5:09 pm: > Background > ------------------ >=20 > Previously if a older kernel runs on a newer firmware, it may enable > all available states irrespective of its capability of handling it. > Consider a case that some stop state has a bug, we end up disabling all > the stop states. This patch introduces selective control to solve this > problem. >=20 > Previous version of these patches can be found at: > https://lkml.org/lkml/2018/10/11/544 > These patch however also had patches for support of opal save-restore > which now I am decoupling and will take them seperately. > I have posted the corresponding skiboot patches for this kernel patchset > here : https://patchwork.ozlabs.org/cover/1144587/ >=20 > What's new? > -------------------- >=20 > Add stop states under ibm,idle-states in addition to the current array > based device tree properties. >=20 > New device tree format adds a compatible flag which has version > corresponding to every state, so that only kernel which has the capabilit= y > to handle the version of stop state will enable it. Drawback of the array > based dt node is that versioning of idle states is not possible. >=20 > Older kernel will still see stop0 and stop0_lite in older format and we > will deprecate it after some time. >=20 > Consider a case that stop4 has a bug. We take the following steps to > mitigate the problem. >=20 > 1) Change compatible string for stop4 in OPAL to "stop4,v2" from > "stop4,v1", i.e. basicallly bump up the previous version and ship the > new firmware. >=20 > 2) The kernel will ignore stop4 as it won't be able to recognize this > new version. Kernel will also ignore all the deeper states because its > possible that a cpu have requested for a deeper state but was never able > to enter into it. But we will still have shallower states that are there > before stop 4. This, thus prevents from completely disabling stop states. >=20 > Linux kernel can now look at the version string and decide if it has the > ability to handle that idle state. Henceforth, if kernel does not know > about a version, it will skip that state and all the deeper state. >=20 > Once when the workaround are implemented into the kernel, we can bump up > the known version in kernel for that state, so that support can be > enabled once again in kernel. >=20 > New Device-tree : >=20 > Final output > power-mgt { > ... > ibm,enabled-stop-levels =3D <0xec000000>; > ibm,cpu-idle-state-psscr-mask =3D <0x0 0x3003ff 0x0 0x3003ff>; > ibm,cpu-idle-state-latencies-ns =3D <0x3e8 0x7d0>; > ibm,cpu-idle-state-psscr =3D <0x0 0x330 0x0 0x300330>; > ibm,cpu-idle-state-flags =3D <0x100000 0x101000>; > ibm,cpu-idle-state-residency-ns =3D <0x2710 0x4e20>; > ibm,idle-states { > stop4 { > flags =3D <0x207000>; > compatible =3D "stop4,v1", > psscr-mask =3D <0x0 0x3003ff>; > latency-ns =3D <0x186a0>; > residency-ns =3D <0x989680>; > psscr =3D <0x0 0x300374>; > ... > }; > ... > stop11 { > ... > compatible =3D "stop11,v1", > ... > }; > }; I'm not sure about this. I think the real problem we have is that the OPAL stop API is not actually implementation independent. Because we *can* selectively disable stop states in the firmware if there is a hardware/firmware problem with them. So we need a way to rev ISA/Book4/OPAL stuff so the kernel won't try to use it if it's incapable. We have that today in dt-cpu-ftrs. POWER9 advertises the "idle-stop" feature. An incompatible implementation could advertise idle-stop-v4. That won't allow individual states to remain back compatible, but if=20 there is a new implementation which changes any behaviour it needs to be=20 incopmatible, even if Linux happens to not rely on said behaviour yet. =20 So I don't think we can keep any of them (except stop0 lite) compatible,=20 so I don't see a real downside (we'll have to discuss this offline). Thanks, Nick =