[Linaro-open-discussions] Notes on Linaro Open Discussions meeting 4 Nov 2020

Lorenzo Pieralisi lorenzo.pieralisi at arm.com
Mon Nov 23 15:12:53 UTC 2020

On Thu, Nov 05, 2020 at 12:36:21PM +0000, Jonathan Cameron via Linaro-open-discussions wrote:
> Hi all,
> This is a fusion of Mike's notes and my own. Please add anything I missed!
> People may well be misidentified (sorry about that).  Was very good active discussion.
> Thanks to all involved and Mike in particular for organizing it and taking live notes.
> General request
>  - Slides for all topics next time to introduce topics as not everyone on call will have necessary
>    background (and those that do might need reminding!) 
>    Hanjun is sending Mike his slides (uncore DVFS) to add to the collaborate page.
> IORT - Reserve memory regions (RMR)
> ===================================
> * Shameer gave summary
>  - IORT Revision E (https://developer.arm.com/documentation/den0049/latest/) introduced new node type.
>  - A way to describe memory regions that should have unity mapping in the SMMU.
>  - Use case is a PCIe RAID card that has FW that uses a pool of host memory (hidden from OS).
> * Status
>  - Patches out for ACPICA
>    - Question raised by ACPICA reviewers on whether spec is final
>    - Spec appears final  (Lorenzo to check) but may be minor unrelated fix in doc to come (Sami).
>  - Patches out for kernel on relevant lists.
>    - Mail from Steven Price (Arm) - (Sami Mujawar who was on the call also involved) interested for EFI
>      framebuffer use case.
> * Open questions
>  - Equivalent from AMD has flag to indicate that unity mapping only needed until driver has taken over
>    (end of kernel boot assumed).  Avoids and issue of holes in address space for VMs.
>    - Huawei not raising this as a requirement, but Lorenzo observed interesting and deserves discussion.
>  - Kexec interaction needs discussions. Steve looking at this an will bring to list.
>  - Lorenzo brought up issue of IORT spec using PCI BDF (stream ID?) which may be reenumerated.
>    - Noted x86 doesn't do this but ARM traditionally does.
>    - There is a DSM that tells the kernel not to reenumerate the PCI bus which ACPI obeys.
>    - Jonathan suggestion was potentially opportunity to cache original stream ID before doing the
>      reenumeration in kernel.
>    - Lorenzo observed we may need a universal solution for all OSes on this.
>      Lorenzo took AI to go away and think about it before next call.
>  - Stalling issues on patch?  Probably only Kexec though should be careful around possible future
>    regressions on the BDF issue (not a blocker)
>  - Related DT story.  Huawei server team not interested as no DT support and can't test.
>    Lorenzo suggested looping in Thierry Reding and reference a patch set
>    (probably https://lore.kernel.org/linux-iommu/20200904130000.691933-1-thierry.reding@gmail.com/#t)
>    Huawei more than happy to have others add the DT support :)
> AI summary:
> * Kexec discussion - on list.
> * Use of BDF discussion - revisit here next time.

I have an update on this topic and the RMR flags to free up IOVA.

Is December 2nd confirmed as next session ? If possible 3PM GMT works
better for me; the NUMA topic raised by Jonathan is another interesting
topic for debate. Other than that we can slot in the topics that
weren't discussed last time:


even though those require a bit of preparation so the sooner we finalize
the schedule the better.

Please let me know, thanks.


> * DT alignment. Don't want different solutions for each firmware type.
> * Lorenzo / Sami to check IORT revision E is final.
> ===
> Zangfei gave summary:
>  - Huawei has devices that are not PCIe but are presented as such.
>  - They support stall mode for SVA (spec violation)
>  - Resistance from kernel maintainers to maintaining a white list for any quirk. Fine to fix
>    it once (JPB), but not to keep doing so.
>  - Note that stall mode not yet supported at all (JPB to send out this cycle).
>  - If longer term fix need add can't be done via PCISIG etc then need to convince
>    PCI and SMMU maintainers.   Noted that quirk is very little code.
> * Other SVA topics.
>   - Mentioned virtual SVA (no actually problems just expressing interest!)
>   - Would need Eric Auger, wasn't on topic list so Eric not on call.
> AI: Nothing planned until after JPB has upstreamed stall mode. Hard to have discussion before that.
> ====
> guohanjun
> Solutions exist for
> * CPU DVFS (voltage + frequency scaling)
> * PCIe device power states etc
> No standard way of controlling Uncore voltage and frequency for ACPI based systems.
> 3 options:
> 1. MMIO / kernel driver.
> 2. PSCI via trusted firmware and system management controller.
> 3. ACPI (wrapping up an op region and SCMI)
> Clarifications / discussions.
>  * Vincent G: Power states, or voltage frequency of interest? Ans Voltage Freq
>  * Considered SCMI?  Ans: Works only for DT as SCMI under ACPI is wrapped up in AML
>    so looks like an ACPI interface.
>  * Sudeep H: Necessary to trace CPU freq?  Yes.
>  * Sudeep H: Why not do it in firmware entirely?  Ans. Not just CPU.  For example PCI device accessing
>    memory may well need the ring bus to be fast.
>  * Vincent G: Bandwidth affected?  Yes. VG: mobile does this by specifying a BW requirement (via SCMI.-
>  * Sudeep H Observed need to expose it via ACPI spec. (option 3 above).
>  * Sudeep H: Does PCI also need fine-grain control? We might need to add to the spec.
>  * Sudeep H:  What are the requirements? gaohanjun: Now we just frequency scaling. 
>  * Jonathan C: Noted PCI power state is not enough.  It's workload dependent.
>  * Sudeep H: We need to gather all the info, need to talk in ASWG about DVFS
>  * Jonathan C:  For now direct control probably makes sense.  Whilst it would be nice to have
>    a detailed enough system description in a standard way to make general software that is a
>    big spec job.
>  * Jonathan C: Seems like true standard SW will not happen any time soon.
> AI: RFC to the linux-pm / linux-acpi Rafael and those in this discussion to ask about 
>     interest in adding per device DVFS to ACPI spec.  Possibly pursue code first ACPI
>     approach.
> If I've miss listed or "volunteered" anyone for AIs they didn't agree to then please
> correct that.
> Thanks all for contributions. I for one found it a very useful call!
> Jonathan
> -- 
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