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

[Conor Dooley]

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.

> > > Signed-off-by: Yu Chien Peter Lin <peterlin@xxxxxxxxxxxxx>
> > > ---
> > > 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@xxxxxxxxxxxxx/#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.

Thanks,
Conor.

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