[RFC 0/6] Sparse HART id support

[Atish Patra]

On Mon, Dec 13, 2021 at 3:11 PM Rob Herring <robh at kernel.org> wrote:
>
> On Mon, Dec 13, 2021 at 3:27 PM Atish Patra <atishp at atishpatra.org> wrote:
> >
> > On Mon, Dec 6, 2021 at 7:28 AM Rob Herring <robh at kernel.org> wrote:
> > >
> > > On Fri, Dec 03, 2021 at 04:20:32PM -0800, Atish Patra wrote:
> > > > Currently, sparse hartid is not supported for Linux RISC-V for the following
> > > > reasons.
> > > > 1. Both spinwait and ordered booting method uses __cpu_up_stack/task_pointer
> > > >    which is an array size of NR_CPUs.
> > > > 2. During early booting, any hartid greater than NR_CPUs are not booted at all.
> > > > 3. riscv_cpuid_to_hartid_mask uses struct cpumask for generating hartid bitmap.
> > > > 4. SBI v0.2 implementation uses NR_CPUs as the maximum hartid number while
> > > >    generating hartmask.
> > > >
> > > > In order to support sparse hartid, the hartid & NR_CPUS needs to be disassociated
> > > > which was logically incorrect anyways. NR_CPUs represent the maximum logical|
> > > > CPU id configured in the kernel while the hartid represent the physical hartid
> > > > stored in mhartid CSR defined by the privilege specification. Thus, hartid
> > > > can have much greater value than logical cpuid.
> > >
> > > We already have a couple of architectures with logical to physical CPU
> > > id maps. See cpu_logical_map. Can we make that common and use it here?
> >
> > Yes. We can move the cpu_logical_map(which is a macro) &
> > __cpu_logical_map(actual array with NR_CPUS size)
> > to common code so that all the architecture can use it instead of
> > defining it separately.
>
> IIRC, the macro is what varies by arch and I would move to static
> inlines rather than supporting:
>
> cpu_logical_map(cpu) = 0xdeadbeef;
>

Sounds good.


> >
> > > That would also possibly allow for common populating the map from DT.
> > >
> >
> > I didn't understand this part. The mapping is populated at run time
> > [1] as the boot cpu can be any hart in RISC-V.
> > That booting hart will be mapped to cpu 0. All others will be mapped
> > based on how the cpu node is laid out in the DT.
> > Do you mean we can move the 2nd part to common code as well ?
>
> Yes, as the DT platforms just loop thru the cpu nodes and fill the
> logical map based on 'reg', I don't think that needs to be per arch
> once we have a common map. But not asking for that now.
>

It would make sense to keep them together in a series. I can take a stab
at it once this series is merged so that we don't end up in
conflicting changes between
two series.

> Rob



-- 
Regards,
Atish