Re: [RFC PATCH v2 07/10] perf: RISC-V: Move T-Head PMU to CPU feature alternative framework

Yu-Chien Peter Lin
2023-10-22T09:10:34Z

Hi Conor,

On Fri, Oct 20, 2023 at 10:05:20AM +0100, Conor Dooley wrote:
> On Fri, Oct 20, 2023 at 04:54:58PM +0800, Yu-Chien Peter Lin wrote:
> > On Thu, Oct 19, 2023 at 05:13:00PM +0100, Conor Dooley wrote:
> > > On Thu, Oct 19, 2023 at 10:01:19PM +0800, Yu Chien Peter Lin wrote:
> > >
> > > $subject: perf: RISC-V: Move T-Head PMU to CPU feature alternative framework
> > >
> > > IMO, this should be "RISC-V, perf:" or just "RISC-V" as the changes
> > > being made to the arch code are far more meaningful than those
> > > elsewhere.
> >
> > OK will update the subject to "RISC-V:"
> >
> > > > The custom PMU extension was developed to support perf event sampling
> > > > prior to the ratification of Sscofpmf. Instead of utilizing the standard
> > > > bits and CSR of Sscofpmf, a set of custom CSRs is added. So we may
> > > > consider it as a CPU feature rather than an erratum.
> > > >
> > > > T-Head cores need to append "xtheadpmu" to the riscv,isa-extensions
> > > > for each cpu node in device tree, and enable CONFIG_THEAD_CUSTOM_PMU
> > > > for proper functioning as of this commit.
> > >
> > > And in doing so, you regress break perf for existing DTs :(
> > > You didn't add the property to existing DTS in-kernel either, so if this
> > > series was applied, perf would just entirely stop working, no?
> >
> > Only `perf record/top` stop working I think.
> >
> > There are too many users out there, and don't have the boards to
> > test, so leave those DTS unchanged, it would be great if T-Head
> > community could help to check/update their DTS.
>
> So, there are too many users to add xtheadpmu to the devicetrees, but
> not too many users to make changes that will cause a regression?
> I'm not following the logic here, sorry.

humm, I'll try. I assume that the sun20i-d1s.dtsi is all I need
to update for T-Head PMU.

> > > > Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
> > > > ---
> > > > Hi All,
> > > >
> > > > This is in preparation for introducing other PMU alternative.
> > > > We follow Conor's suggestion [1] to use cpu feature alternative
> > > > framework rather than errta, if you want to stick with errata
> > > > alternative or have other issues, please let me know. Thanks.
> > >
> > > Personally, I like this conversion, but it is going to regress support
> > > for perf on any T-Head cores which may be a bitter pill to get people to
> > > actually accept...
> > > Perhaps we could add this "improved" detection in parallel, and
> > > eventually remove the m*id based stuff in the future.
> > >
> > > > [1] https://patchwork.kernel.org/project/linux-riscv/patch/20230907021635.1002738-4-peterlin@andestech.com/#25503860
> > > >
> > > > Changes v1 -> v2:
> > > > - New patch
> > > > ---
>
> > > > @@ -805,7 +816,8 @@ static int pmu_sbi_setup_irqs(struct riscv_pmu *pmu, struct platform_device *pde
> > > > if (riscv_isa_extension_available(NULL, SSCOFPMF)) {
> > > > riscv_pmu_irq_num = RV_IRQ_PMU;
> > > > riscv_pmu_use_irq = true;
> > > > - } else if (IS_ENABLED(CONFIG_ERRATA_THEAD_PMU) &&
> > > > + } else if (riscv_isa_extension_available(NULL, XTHEADPMU) &&
> > > > + IS_ENABLED(CONFIG_THEAD_CUSTOM_PMU) &&
> > > > riscv_cached_mvendorid(0) == THEAD_VENDOR_ID &&
> > > > riscv_cached_marchid(0) == 0 &&
> > > > riscv_cached_mimpid(0) == 0) {
> > >
> > > Can all of the m*id checks be removed, since the firmware is now
> > > explicitly telling us that the T-Head PMU is supported?
> >
> > I can only comfirm that boards with "allwinner,sun20i-d1" compatible
> > string uses the T-Head PMU device callbacks.
>
> I'm not sure how that is an answer to my question.

Sorry for that unclear answer.
Yes, I agree we no longer need to check the m*id here.

In OpenSBI, it appears that allwinner D1 is the only platform that
has T-Head PMU support, the other T-Head platforms need to ensure
that the callbacks [1] are registered in order to work with SBI
PMU driver in kernel.

Regards,
Peter Lin

[1] https://github.com/riscv-software-src/opensbi/blob/v1.3.1/platform/generic/allwinner/sun20i-d1.c#L263-L272

> Thanks,
> Conor.