Re: [PATCH 03/11] dt-bindings: interrupt-controller: RISC-V PLIC documentation

From: Atish Patra
Date: Wed Aug 08 2018 - 02:42:10 EST

On 8/7/18 7:17 PM, Palmer Dabbelt wrote:
On Mon, 06 Aug 2018 13:59:48 PDT (-0700), robh+dt@xxxxxxxxxx wrote:
On Thu, Aug 2, 2018 at 4:08 PM Atish Patra <atish.patra@xxxxxxx> wrote:

On 8/2/18 4:50 AM, Christoph Hellwig wrote:
From: Palmer Dabbelt <palmer@xxxxxxxxxxx>

This patch adds documentation for the platform-level interrupt
controller (PLIC) found in all RISC-V systems. This interrupt
controller routes interrupts from all the devices in the system to each
hart-local interrupt controller.

Note: the DTS bindings for the PLIC aren't set in stone yet, as we might
want to change how we're specifying holes in the hart list.

Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxx>
[hch: various fixes and updates]
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
.../interrupt-controller/sifive,plic0.txt | 57 +++++++++++++++++++
1 file changed, 57 insertions(+)
create mode 100644 Documentation/devicetree/bindings/interrupt-controller/sifive,plic0.txt

diff --git a/Documentation/devicetree/bindings/interrupt-controller/sifive,plic0.txt b/Documentation/devicetree/bindings/interrupt-controller/sifive,plic0.txt
new file mode 100644
index 000000000000..c756cd208a93
--- /dev/null
+++ b/Documentation/devicetree/bindings/interrupt-controller/sifive,plic0.txt
@@ -0,0 +1,57 @@
+SiFive Platform-Level Interrupt Controller (PLIC)
+SiFive SOCs include an implementation of the Platform-Level Interrupt Controller
+(PLIC) high-level specification in the RISC-V Privileged Architecture
+specification. The PLIC connects all external interrupts in the system to all
+hart contexts in the system, via the external interrupt source in each hart.
+A hart context is a privilege mode in a hardware execution thread. For example,
+in an 4 core system with 2-way SMT, you have 8 harts and probably at least two
+privilege modes per hart; machine mode and supervisor mode.
+Each interrupt can be enabled on per-context basis. Any context can claim
+a pending enabled interrupt and then release it once it has been handled.
+Each interrupt has a configurable priority. Higher priority interrupts are
+serviced first. Each context can specify a priority threshold. Interrupts
+with priority below this threshold will not cause the PLIC to raise its
+interrupt line leading to the context.
+While the PLIC supports both edge-triggered and level-triggered interrupts,
+interrupt handlers are oblivious to this distinction and therefore it is not
+specified in the PLIC device-tree binding.
+While the RISC-V ISA doesn't specify a memory layout for the PLIC, the
+"sifive,plic0" device is a concrete implementation of the PLIC that contains a
+specific memory layout, which is documented in chapter 8 of the SiFive U5
+Coreplex Series Manual <>.
+Required properties:
+- compatible : "sifive,plic0"

I think there was a thread bouncing around somewhere where decided to pick the
official name of the compatible string to be "sifive,plic-1.0". The idea here
is that the PLIC is compatible across all of SiFive's current implementations,
but there's some limitations in the memory map that will probably cause us to
spin a version 2 at some point so we want major version number. The minor
number is just in case we find some errata in the PLIC.

+- #address-cells : should be <0>
+- #interrupt-cells : should be <1>
+- interrupt-controller : Identifies the node as an interrupt controller
+- reg : Should contain 1 register range (address and length)

The one in the real device tree has two entries.
reg = <0x00000000 0x0c000000 0x00000000 0x04000000>;

Is it intentional or just incorrect entry left over from earlier days?

+ reg = <0xc000000 0x4000000>;

Looks to me like one has #size-cells and #address-cells set to 2 and
the example is using 1.

Yes. For some background on how this works: we have a hardware generator that
has a tree-of-busses abstraction, and each device is attached to some point on
that tree. When a device gets attached to the bus, we also generate a device
tree entry. For whatever system I generated the example PLIC device tree entry
from, it must have been attached to a bus with addresses of 32 bits or less,
which resulted in #address-cells and #size-cells being 1.

Thanks Palmer for the detailed explanation.

Christoph has a HiFive Unleashed, which has a fu540-c000 on it, which is
probably not what I generated as an example -- the fu540-c000 is a complicated
configuration, when I mess around with the hardware I tend to use simple ones
as I'm not really a hardware guy. I suppose the bus that the PLIC is hanging
off on the fu540-c000 has addresses wider than 32 bits. This makes sense, as
the machine has 8GiB of memory and the PLIC is on a bus that's closer to the
core than the DRAM is, so it'd need at least enough address bits to fit 8GiB.

Is the inconsistency a problem? I generally write device tree handling code to
just respect whatever #*-fields says and don't consider that part of the
specification of the binding. I don't mind changing the example to have
#size-fields and #address-fields to be 2, but since it's not wrong I also don't
see any reason to change it. We do have 32-bit devices with PLICs, and while
they're not Linux-capable devices we're trying to adopt the Linux device tree
bindings through the rest of the RISC-V software ecosystem as they tend to be
pretty well thought out.

Sounds good to me. IMHO, the inconsistencies and its reasoning are well documented which is good enough for now.