Re: [patches] Re: [PATCH] dt-bindings: Add an enable method to RISC-V

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On Tue, 21 Nov 2017 03:04:52 PST (-0800), mark.rutland@xxxxxxx wrote:
Hi Palmer,

On Mon, Nov 20, 2017 at 11:50:22AM -0800, Palmer Dabbelt wrote:
RISC-V doesn't currently specify a mechanism for enabling or disabling
CPUs.  Instead, we assume that all CPUs are enabled on boot, and if
someone wants to save power we instead put a CPU to sleep via a WFI
loop.

This patch adds "enable-method" to the RISC-V CPU binding, which
currently only has the value "none".  This allows us to change the
enable method in the future.

I think you might want to be a bit more explicit about what this means,
and this could do with a better name, as "none" sounds like the CPU is
unusable, rather than it having been placed within the kernel already by
the FW/bootloader (which IIUC is what happens currently).

It was proposed to make "enable-method" optional, and have the lack of an enable method signify the current scheme. The current scheme is that the bootloader starts every hart at the kernel's entry point.

Calling this "always-enabled" was also suggested, which seems fine to me.

As previosuly commented, I also really think you'll want to define a
simple boot protocol (like PPC spin-table) whereby the kernel can bring
each CPU into the kernel independently. That will save you a lot of pain
in future with things like kexec, suspend/resume, etc.

For arm64 we had a spin-table clone (implemented in our boot-wrapper
firmware) that allowed us to bring CPUs into the kernel explicitly.
However, we made the mistake of allowing CPUs to share a mailbox, and we
couldn't tell how many CPUs were stuck in the kernel at any point in
time (rendering kexec, suspend, etc impossible).

This is actually why I'm kind of pushing back on this: because we don't know how we're actually going to handle this, I don't want to go build an interface to the firmware that might be broken. Essentially what we're doing now is just keeping the spin table entirely within Linux, so we can change this interface whenever we want. The start of our kernel looks like

 _start(char *dtb_pointer, long hartid)
   if (atomic_increment_return(hart_lottery) == 0)
     start_kernel()
   else
     while (READ_ONCE(__cpu_up_has_turned_on_hart[hartid]) == 0)
       wait_for_interrupt()
     smp_callin()

If I understand correctly, this is essentially what the spin tables are doing in arm64. Our mechanism is a bit different because we can expose a much more complicated interface here, but since the interface can change (it's a kernel-internal interface, not a firmware->kernel interface) that's the natural thing to do.

While I haven't actually gone through and looked at any of this (and I admit I have only a vague idea of how it works), I think this should work fine for kexec, CPU hotplug, and suspend. kexec is easy: the fresh kernel's image will boot exactly like a regular one, as all the harts can just jump to the entry point at the same time. Since "hart_lottery" is initialized to 0 by the ELF there isn't anything special required to make it work.

Actually turning off harts will require us to add an interface that does so, which will probably happen via an SBI call. We haven't actually designed the interface yet, but I'm assuming it'll just reset the hart. In general, we like to make any interface that sleeps also work as a NOP, so for now let's just pretend that this interface does nothing and go straight to_start. This should map pretty well, our __cpu_down could just be the mirror of __cpu_up

 __cpu_down(int hartid)
   __cpu_up_has_turned_on_hart[hartid] = false;
   atomic_decrement(hart_lottery);
   __sbi_suspend_hart();
   jump _start

That should cover hotplug, and then suspend is just a matter of hotplugging out the last CPU. I assume that lots of our stuff will blow up when we start removing harts at runtime, but that'll all happen regardless of how we wake them up. There's also a bit of a race here (bringing up a hart while the last one is suspending), and that counter overflows, but those seem solvable.

Does that sound sane? If not, I'd be happy to go and design a spin table firmware interface. We just like to avoid inventing external interfaces until we really know what we're doing :).

Thanks,
Mark.

CC: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxx>
---
 Documentation/devicetree/bindings/riscv/cpus.txt | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/devicetree/bindings/riscv/cpus.txt b/Documentation/devicetree/bindings/riscv/cpus.txt
index adf7b7af5dc3..dd9e1ae197e2 100644
--- a/Documentation/devicetree/bindings/riscv/cpus.txt
+++ b/Documentation/devicetree/bindings/riscv/cpus.txt
@@ -82,6 +82,11 @@ described below.
                 Value type: <string>
                 Definition: Contains the RISC-V ISA string of this hart.  These
                             ISA strings are defined by the RISC-V ISA manual.
+        - cpu-enable-method:
+		Usage: required
+		Value type: <stringlist>
+		Definition: Must be one of
+			"none": This CPU's state cannot be changed.

 Example: SiFive Freedom U540G Development Kit
 ---------------------------------------------
@@ -105,6 +110,7 @@ Linux is allowed to run on.
                         reg = <0>;
                         riscv,isa = "rv64imac";
                         status = "disabled";
+                        enable-method = "none";
                         L10: interrupt-controller {
                                 #interrupt-cells = <1>;
                                 compatible = "riscv,cpu-intc";
@@ -130,6 +136,7 @@ Linux is allowed to run on.
                         reg = <1>;
                         riscv,isa = "rv64imafdc";
                         status = "okay";
+                        enable-method = "none";
                         tlb-split;
                         L13: interrupt-controller {
                                 #interrupt-cells = <1>;
--
2.13.6

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