linux-kernel - Re: [PATCH] drivers/perf: riscv_pmu_sbi: add support for PMU variant on T-Head C9xx cores

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Date:   Thu, 25 Aug 2022 04:04:43 +0200
From:   Heiko Stuebner <heiko@...ech.de>
To:     Atish Patra <atishp@...shpatra.org>,
        Anup Patel <apatel@...tanamicro.com>
Cc:     anup@...infault.org, will@...nel.org, mark.rutland@....com,
        paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, philipp.tomsich@...ll.eu,
        cmuellner@...ux.com
Subject: Re: [PATCH] drivers/perf: riscv_pmu_sbi: add support for PMU variant on T-Head C9xx cores

Am Donnerstag, 18. August 2022, 10:24:33 CEST schrieb Anup Patel:
> On Thu, Aug 18, 2022 at 1:03 AM Atish Patra <atishp@...shpatra.org> wrote:
> >
> > On Wed, Aug 17, 2022 at 4:13 AM Heiko Stuebner <heiko@...ech.de> wrote:
> > >
> > > With the T-HEAD C9XX cores being designed before or during the ratification
> > > to the SSCOFPMF extension, they implement functionality very similar but
> > > not equal to it. So add some adaptions to allow the C9XX to still handle
> > > its PMU through the regular SBI PMU interface instead of defining new
> > > interfaces or drivers.
> > >
> >
> > IMO, vendor specific workarounds in the generic implementation is not
> > a good idea.
> > If we have to support it, I think we should just put the IRQ number in
> > the DT and parse from the DT.
> > The initial sscofpmf series was based on the DT. It was removed later
> > as there was no need for it at that time.
> > We can always revive it.
> >
> > Regarding the CSR number difference and static key enablement, can we
> > use code patching techniques here as well ?
> > At least all the T-HEAD C9XX core erratas are in one place.
> >
> > The alternate would be just to say T-HEAD C9XX support SSCOFPMF but
> > with erratas. I don't prefer this approach
> > but it keeps the vendor specific checks out of the generic code.
> 
> Whether to have a DT node (or not) was already discussed and concluded
> in the past.
> 
> We don't need a DT node just to get the IRQ number. The T-HEAD custom
> IRQ number can be derived based on mvendorid.

Yeah, I remember reading that discussion and thus went with the mvendorid
way in this patch.

> Also, all these T-HEAD specific changes in SBI PMU driver should be
> implemented as erratas using ALTERNATIVE() macros.

(1) "All these T-HEAD specific changes ..."
Actually the only T-HEAD-specific change is reading that different CSR
for the overflow information, the rest only makes the irq-number variable

(2) ALTERNATIVE macros are working on assembler instructions, so are you
sugesting to replace the generic csr_read() for the overflow-csr with a
custom copy like

#define sbi_pmu_read_overflow(void)						\
({								\
	register unsigned long __v;				\
	ALT_THEAD_PMU_OVERFLOW(__v); \
	__v;							\
})

and then in errata_list.h

#define ALT_THEAD_PMU_OVERFLOW(__ovl) \
__asm__ __volatile__ (alternative(
	"csrr %0, " __ASM_STR(CSR_SSCOUNTOVF),	\
	"csrr %0, " __ASM_STR(THEAD_C9XX_CSR_SCOUNTEROF), THEAD_VENDOR_ID, \
		ERRATA_THEAD_PMU, CONFIG_ERRATA_THEAD_PMU) \
			      : "=r" (__ovl) :			\
			      : "memory");

I'm not yet seeing what you're gaining by going with this approach:
- that the overflow-csr is the same bitwise but only at a different
  address is specific to the c9xx, other deviants might implement things
  completely different
- you're not getting rid of the thead mention
- and we're now duplicating the generic csr-read code

Is this the preferred way or what am I overlooking?

Thanks
Heiko