linux-kernel - Re: [PATCH 1/1] arm: topology: parse the topology from the dt

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Message-ID: <CAG7+-3P6FYhqmt3p6g8b5ZXzaFW2pfx-vOZJYwuLCKXVqfFMVw@mail.gmail.com>
Date:   Wed, 14 Apr 2021 19:26:48 +0800
From:   Ruifeng Zhang <ruifeng.zhang0110@...il.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux@...linux.org.uk, sudeep.holla@....com,
        Greg KH <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>, a.p.zijlstra@...llo.nl,
        mingo@...nel.org, ruifeng.zhang1@...soc.com, nianfu.bai@...soc.com,
        linux-arm-kernel@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] arm: topology: parse the topology from the dt

Dietmar Eggemann <dietmar.eggemann@....com> 于2021年4月14日周三 下午5:43写道：
>
> On 13/04/2021 15:26, Ruifeng Zhang wrote:
> > Thanks for your review. Patch-v2 that solve the capacity issue will be
> > uploaded as soon as possible. : )
> >
> > Valentin Schneider <valentin.schneider@....com> 于2021年4月13日周二 下午7:40写道：
> >>
> >> On 13/04/21 14:13, Ruifeng Zhang wrote:
> >>> Valentin Schneider <valentin.schneider@....com> 于2021年4月12日周一 下午11:33写道：
> >>>> I'm not fluent at all in armv7 (or most aarch32 compat mode stuff), but
> >>>> I couldn't find anything about MPIDR format differences:
> >>>>
> >>>>   DDI 0487G.a G8.2.113
> >>>>   """
> >>>>   AArch32 System register MPIDR bits [31:0] are architecturally mapped to
> >>>>   AArch64 System register MPIDR_EL1[31:0].
> >>>>   """
> >>>>
> >>>> Peeking at some armv7 doc and arm/kernel/topology.c the layout really looks
> >>>> just the same, i.e. for both of them, with your example of:
> >>>
> >>> The cortex-a7 spec DDI0464F 4.3.5
> >>> https://developer.arm.com/documentation/ddi0464/f/?lang=en
> >>>
> >>
> >> Ah, so that's where the core_id=bit[1:0] comes from. That does still
> >> conform to the MPIDR format, and as you point out below that's being parsed
> >> the same (aff2, aff1, aff0) == mpidr([23:16][15:8][7:0])
> >>
> >>> The current arch/arm/kernel/topology code parse the MPIDR with a armv7 format.
> >>> the parse code is:
> >>> void store_cpu_topology(unsigned int cpuid)
> >>> {
> >>>     ...
> >>>     cpuid_topo->thread_id = -1;
> >>>     cpuid_topo->core_id = MPIDR_AFFINITY_LEVEL(mpidr, 0);
> >>>     cpuid_topo->package_id = MPIDR_AFFINITY_LEVEL(mpidr, 1);
> >>>     ...
> >>> }
> >>>>
> >>>>   core0: 0000000080000000
> >>>>   core1: 0000000080000100
> >>>>   core2: 0000000080000200
> >>>>   ...
> >>>>
> >>>> we'll get:
> >>>>
> >>>>   |       | aff2 | aff1 | aff0 |
> >>>>   |-------+------+------+------|
> >>>>   | Core0 |    0 |    0 |    0 |
> >>>>   | Core1 |    0 |    1 |    0 |
> >>>>   | Core2 |    0 |    2 |    0 |
> >>>>       ...
> >>>>
> >>>> Now, arm64 doesn't fallback to MPIDR for topology information anymore since
> >>>>
> >>>>   3102bc0e6ac7 ("arm64: topology: Stop using MPIDR for topology information")
> >>>>
> >>>> so without DT we would get:
> >>>>   |       | package_id | core_id |
> >>>>   |-------+------------+---------|
> >>>>   | Core0 |          0 |       0 |
> >>>>   | Core1 |          0 |       1 |
> >>>>   | Core2 |          0 |       2 |
> >>>>
> >>>> Whereas with an arm kernel we'll end up parsing MPIDR as:
> >>>>   |       | package_id | core_id |
> >>>>   |-------+------------+---------|
> >>>>   | Core0 |          0 |       0 |
> >>>>   | Core1 |          1 |       0 |
> >>>>   | Core2 |          2 |       0 |
> >>>>
> >>>> Did I get this right? Is this what you're observing?
> >>>
> >>> Yes, this is a problem if an armv8.2 or above cpu is running a 32-bit
> >>> kernel on EL1.
> >>
> >>
> >> With the above MPIDR(_EL1) values, you would have the same problem in
> >> aarch64 mode on any kernel predating
> >>
> >>   3102bc0e6ac7 ("arm64: topology: Stop using MPIDR for topology information")
> >>
> >> since all Aff0 values are 0. Arguably those MPIDR(_EL1) values don't
> >> make much sense (cores in the same cluster should have different Aff0
> >> values, unless SMT), but in arm64 that's usually "corrected" by DT.
> >>
> >> As you pointed out, arm doesn't currently leverage the cpu-map DT entry. I
> >> don't see any obvious problem with adding support for it, so if you can fix
> >> the capacity issue Dietmar reported, I think we could consider it.
>
> Coming back to your original patch. You want to use parse_dt_topology()
> from drivers/base/arch_topology.c to be able detect a cpu-map in dt and
> so bypassing the read of mpidr in store_cpu_topology()?

Yes
>
> Looks like sc9863a has two frequency domains (1.6 and 1.2GHz). So
> technically it's a big.LITTLE system (based only on max CPU frequency
> (not on uarch) differences).
> But the dts file doesn't contain any `capacity-dmips-mhz` entries? So
> asymmetric CPU capacity (even only based on max CPU frequency) detection
> won't kick in. Since you don't have any uarch diffs, you would have to
> specify `capacity-dmips-mhz = <1024>` for each CPU.

Yes, for capacity, the DT should have a capacity-dmips-MHz entry or a
clock-frequency entry (for A7 and A15 only).
The sc9863a dts is a vendor file,  in my opinion is not appropriate to
be update with this series.
What do you think if I independently update the sc9863a dts file later?