On Thu, Oct 28, 2021 at 5:07 PM Jessica Clarke [off-list ref] wrote:

On 29 Oct 2021, at 00:37, Atish Patra [off-list ref] wrote:

quoted

On Thu, Oct 28, 2021 at 1:49 PM Jessica Clarke [off-list ref] wrote:

quoted

On Mon, Oct 25, 2021 at 12:53:49PM -0700, Atish Patra wrote:

quoted

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 <redacted>
---
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.

All RISC-V hardware provides the CSRs, they’re part of the privileged
spec and not marked optional. How many aren’t hard-wired to zero is up
to the implementation. But even then you can’t know from the hardware
alone what is supported; the firmware has to enable S-mode (and
U-mode)’s ability to read them, so you can’t assume anything in a
static device tree hard-coded in Linux about what firmware has done.
Since you currently have to query the firmware to determine what’s
available to you anyway, I see no benefit from having a node in the
device tree that tells you your firmware *might* have counters you can
use.

quoted

Additionally, the counter overflow interrupt number needs to be
defined through the DT as well
so that a clean platform driver can be implemented.

The interrupt number is specified as 13 by the Sscofmpf spec.
But that’s not relevant here, the FU740 predates and doesn’t implement
Sscofmpf, meaning there is no interrupt to even define here. And as I
said on the other patch, don’t conflate “SBI PMU firmware interface is
supported” and “Sscofmpf is implemented in the hardware”; the former
should be discovered by talking to firmware, and the latter should be
discovered like any other extension (however that ends up happening).

Presence of sscof extension can be discovered through general extension
discovery mechanism (probably a separate DT node..that's a separate discussion).

However, the interrupt number discovery has to be through DT so the
platform driver
can probe the DT to figure out that.

quoted

quoted

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.

You don’t need a device tree for that; that same code path can just be
“use the existing standard firmware interface”. That also has the
benefit that it’s not tied to device tree and so works identically for
ACPI, rather than needing an ACPI version of it.

I don't disagree with that argument. However, we need a DT node for
interrupt number as explained in the above.
A DT based platform driver allows us to provide a unified code path
which can handle both kinds of platforms described below.

1. Platforms without sscof extension
2. Platforms with sscof extension that requires a DT node for interrupt number

Otherwise, the driver has to do the following things in order.

1. Probe PMU extension
2. first check if sscof extension is present in the special RISC-V ISA
extension DT node (which is yet to finalize)
3. If sscof extension is present then register for a DT based platform driver.
4. Otherwise, register a simple platform driver.

I am not completely opposed to doing that if there is a strong
technical issue with the current approach.

I see nothing here that can’t be discovered through pre-existing means.
If it can be discovered without use of the device tree then it does not
belong in the device tree; the device tree is purely for things that
cannot otherwise be discovered.

Jess

-- 
Regards,
Atish