Re: [PATCH v6 00/34] arm_mpam: Add basic mpam driver

[Punit Agrawal <punit.agrawal@oss.qualcomm.com>]

Hi Ben, James

Ben Horgan <ben.horgan@arm.com> writes:

> Hi all,
>
> Just a handful of minor changes based on v5 review comments. See individual
> patches for change lgos. I haven't asked James to host any branches for this
> revision but please let me know if this would be helpful to anyone. Thanks for
> all the help with reviews and testing.
>
> Previous cover letter from James:
>
> This is just enough MPAM driver for ACPI. DT got ripped out. If you need DT
> support - please share your DTS so the DT folk know the binding is what is
> needed.
> This doesn't contain any of the resctrl code, meaning you can't actually drive it
> from user-space yet. Because of that, its hidden behind CONFIG_EXPERT.
> This will change once the user interface is connected up.
>
> This is the initial group of patches that allows the resctrl code to be built
> on top. Including that will increase the number of trees that may need to
> coordinate, so breaking it up make sense.
>
> The locking got simplified, but is still strange - this is because of the 'mpam-fb'
> firmware interface specification that is still alpha. That thing needs to wait for
> an interrupt after every system register write, which significantly impacts the
> driver. Some features just won't work, e.g. reading the monitor registers via
> perf.
>
> I've not found a platform that can test all the behaviours around the monitors,
> so this is where I'd expect the most bugs.
>
> The MPAM spec that describes all the system and MMIO registers can be found here:
> https://developer.arm.com/documentation/ddi0598/db/?lang=en
> (Ignored the 'RETIRED' warning - that is just arm moving the documentation around.
>  This document has the best overview)
>
> The expectation is this will go via the arm64 tree.
>
> This series is based on v6.18-rc4, and can be retrieved from: (no v6 version)
> https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git mpam/driver/v5
>
> The rest of the driver can be found here: (no v6 version)
> https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git mpam/snapshot/v6.18-rc4-v5
>
> What is MPAM? Set your time-machine to 2020:
> https://lore.kernel.org/lkml/20201030161120.227225-1-james.morse@arm.com/

Although a little late to the party, I've managed to throw together
enough firmware to describe the MPAM hardware and take this set (more
specifically mpam/snapshot/v6.18-rc4-v5 branch from James' repository)
for a spin. Using the branch, the kernel is able to probe the hardware
and discover the advertised features. Yay! We are in business.

Having said that, there are a few quirks of the platform that run into
issues with later patches in the branch. The platform has MSCs attached
to shared L2 caches which are being skipped during later stages of
initialisation. IIUC, the L2 MSCs' limitations stems from the
assumptions in the resctrl interface.

I was wondering if there are any patches available to relax these
limitations? I can give them a try. Or do these need to be put together
from the ground up? Any pointers greatly appreciated.

Thanks,
Punit

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