Re: [v4 10/11] riscv: dts: fu740: Add pmu node

[Atish Patra <atishp@atishpatra.org>]

On Thu, Oct 28, 2021 at 1:49 PM Jessica Clarke <jrtc27@jrtc27.com> wrote:
>
> On Mon, Oct 25, 2021 at 12:53:49PM -0700, Atish Patra wrote:
> > HiFive unmatched supports HPMCounters but does not implement mcountinhibit
> > or sscof extension. Thus, perf monitoring can be used on the unmatched
> > board without sampling.
> >
> > Add the PMU node with compatible string so that Linux perf driver can
> > utilize this to enable PMU.
> >
> > Signed-off-by: Atish Patra <atish.patra@wdc.com>
> > ---
> >  arch/riscv/boot/dts/sifive/fu740-c000.dtsi | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/riscv/boot/dts/sifive/fu740-c000.dtsi b/arch/riscv/boot/dts/sifive/fu740-c000.dtsi
> > index abbb960f90a0..b35b96b58820 100644
> > --- a/arch/riscv/boot/dts/sifive/fu740-c000.dtsi
> > +++ b/arch/riscv/boot/dts/sifive/fu740-c000.dtsi
> > @@ -140,6 +140,9 @@ soc {
> >               #size-cells = <2>;
> >               compatible = "simple-bus";
> >               ranges;
> > +             pmu {
> > +                     compatible = "riscv,pmu";
> > +             };
>
> This is a property of the user-replaceable firmware, not a property of
> the hardware,

It's a property of hardware that indicates that the hardware supports PMU.
Additionally, the counter overflow interrupt number needs to be
defined through the DT as well
so that a clean platform driver can be implemented.


so having this in the device tree under /soc, let alone
> hard-coded in Linux, is utterly wrong. Why can this not just be probed
> like any other SBI interface? The "Probe SBI extension" interface is
> precisely for this kind of thing.
>
SBI extension is anyways probed to verify if the firmware has PMU
extension or not.
However, adding the DT property allows different platforms (with or
without sscof extension)
to use the same code path.

> Jess
>
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-- 
Regards,
Atish