linux-kernel - Re: [PATCH v3] drivers/perf: riscv_pmu_sbi: add support for PMU variant on T-Head C9xx cores

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Date:   Mon, 05 Sep 2022 20:59:53 +0200
From:   Heiko Stübner <heiko@...ech.de>
To:     Andreas Schwab <schwab@...e.de>
Cc:     atishp@...shpatra.org, 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, Conor.Dooley@...rochip.com
Subject: Re: [PATCH v3] drivers/perf: riscv_pmu_sbi: add support for PMU variant on T-Head C9xx cores

Hi Andreas,

Am Montag, 5. September 2022, 16:30:48 CEST schrieb Andreas Schwab:
> On Sep 05 2022, Heiko Stuebner wrote:
> > To work properly, this requires a matching change in SBI, though the actual
> > interface between kernel and SBI does not change.
> 
> What happens if you mix different kernel and SBI versions?

The interface kernel <-> sbi itself is not changed at all and this of
course only matters to t-head c9xx cpu cores, so I guess we have
the cases:

- non-t-head core:
  no behaviour change independent of versions


- t-head core with everything "old":
  would just uses the regular sbi pmu setup, but as the necessary
  sbi-side pmu config for the c9xx isn't set from u-boot
  (dt-properties mapping sbi-events to the values needed to be written
   to mhpmevent*), this is broken anyway with standard sbi
  

- t-head core with "old" kernel, "new" sbi:
  kernel does not detect the extended features, so should fall back
  to just use the standard pmu features
  

- t-head core with "new" kernel", "old" sbi:
  Same as everything "old", pmu isn't setup correctly in sbi anyway
  for the c9xx at the moment


Heiko