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

From: Palmer Dabbelt
Date: Wed Aug 08 2018 - 15:38:15 EST

On Wed, 08 Aug 2018 07:16:14 PDT (-0700), robh+dt@xxxxxxxxxx wrote:
On Tue, Aug 7, 2018 at 8:17 PM Palmer Dabbelt <palmer@xxxxxxxxxxx> 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 <https://static.dev.sifive.com/U54-MC-RVCoreIP.pdf>.
>> > +
>> > +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.

Is 1.0 an actual version number corresponding to an exact, revision
controlled version of the IP or just something you made up? Looks like
the latter to me and I'm not a fan of s/w folks making up version
numbers for h/w. Standard naming convention is <vendor>,<soc>-<block>
unless you have good reason to deviate (IP for FPGAs where version
numbers are exposed to customers is one example).

The hardware versioning scheme calls it "riscv,plic0", which is what we were originally using. This PLIC isn't actually defined as a RISC-V spec, so we wanted to change it to a "sifive,*" name instead. Since we were going to change the compat string anyway, I thought we'd just introduce a minor number to be safe. Since "0.0" is an awkward number, I figured "1.0" would be saner.

I don't have a concrete idea of when the minor number would be used in the PLIC, but we do have a UART and I'd like to make a minor revision of that. This might be too much detail, but essentially the UART consists of two parts: a byte-wide FIFO that runs at the processor's clock, and then a bit-wide shift register that ruts at the UART clock. The shift register is driven by a simple digital clock divider off the FIFO's clock, which means that whenever you change the FIFO's clock speed (for power management or whatever) you also need to change the clock divider to keep the UART's baud rate constant.

As a result, if you change the clock while the UART is in the middle of transmitting a byte then you get corruption. There's a signal that says "the UART TX queue is empty" that can be read from software, but that signal points to the TX FIFO and doesn't account for the additional time to stream out the contents of the shift register. There are configurations of the baud rate, bus latency, and clock speeds such that the "TX FIFO empty" poll can make it back to the core and the core's write to the clock controller can materialize at whatever magic makes the clocks change before the UART has serialized out every bit in the shift register, which manifests as corruption.

With the current UART hardware, which has a tentative compat string of "sifive,uart0", we need to determine that the shift registers has drained in an open-loop manner (ie, just wait for a bit). This is ugly. I'd like to spin a minor version of the UART that just has an extra control bit made available to software that tracks when the shift register drains, but since the drivers for the old version would be compatible it seems like calling that "sifive,uart1" is too big of a version jump. Of course the Linux OF infrastructure assigns no semantic meaning to compat strings so it doesn't matter here, but we use device trees all over the place
Thus, I thought that if we were going to change the naming scheme that we might as well go put in a minor version number just to be safe. Sorry if that's a bit too much info... :)

Of course, this is all open source RTL so you can just see what's going on here in the PLIC

https://github.com/freechipsproject/rocket-chip/blob/384096a6a73f0e94d6f7bd4bc9cc422e0a213e88/src/main/scala/devices/tilelink/Plic.scala#L69

and in the UART

https://github.com/sifive/sifive-blocks/blob/master/src/main/scala/devices/uart/UART.scala#L91

We'll change the name generated by the hardware to match whatever's decided on here, so there won't be a rift between the hardware and the software.

This actually brings up an issue with the standard naming scheme, which in this cause would be something like "sifive,u540-c000-plic": the RTL is open source, so anyone can go build a chip with it at any time. I've been operating under the impression that it will be impossible to maintain a central database of all implementations, so the fallback name will be the defacto name that becomes the interface.

I'd be happy to put a "sifive,u540-c000-plic" in there (I'd called in "sifive,u540-c000,plic" before, but that's just me not knowing the standard naming scheme) just to make sure we have a specific name. We at SiFive can make sure we provide sane unique names for all our implementations, but I think we also really need to hammer out what the general compat string is because I don't think we can rely one everyone to do that.

There are devices that are very specific to a chip, and there we will have chip-specific names for them -- for example the PRCI (power, reset, clock and interrupt) block is different for pretty much every chip, so it'll be called something like "sifive,u540-c000-prci" (or maybe "sifive,aloe-prci", depending on whether we want to use engineering code names or marketing names in the bindings, which is a debate for later). I'm less worried about these, though, as people building chips must pay a lot of attention to things like clock muxing so they're unlikely to accidentally end up with one floating around.

And defining a version 2 when you find a quirk doesn't work. You've
already shipped the DT. You need to be able to fix issues with just an
OS update. This is why you are supposed to define a compatible string
for each and every SoC (and use a fallback when they are "the
same"TM).

Yes, of course -- we actually put the DTB in ROM on the chips, so there's really no option to change it aside from hacking up something nasty in the bootloader. We're doing this for the u540-c000 because we haven't standardized the bindings. There aren't that many u540-c000s in the wild so I'm OK shouldering the burden of making everyone upload their bootloaders with whatever workarounds are necessary to boot an upstream kernel.

I view that as just my punishment for not having properly discussed the bindings before shipping hardware (and out of tree kernels), but that won't be viable in the future. I view this as just like any other ABI stabilization, where it's not officially stable until we're upstream. That's why I'd like to get a good scheme for naming these in the future, as once this is in I'm stuck with any mistakes forever.

>> > +- #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.

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.

The example is just an example and dts files can use either. For dts
files though, you should use the smallest size necessary and utilize
'ranges'. Some folks seem to think a 64-bit chip needs 64-bit address
and size everywhere. That's true at the top level typically, but
individual buses often don't span more than 4GB of address space. But
all that's out of scope of the example.

OK, thanks. I think we're doing this where it's possible -- we emit ranges for busses, and I know that at least some of them (ie, the low-speed peripheral bus that essentially always fits into a 32-bit physical address) ends up with #{address,size}-cells=1.

I'll try to keep this in mind as we start to submit more bindings.

There are no "Linux device tree bindings". There are DT bindings that
happen to be hosting within the Linux tree for convenience.

Ah, OK. By "Linux device tree bindings" I meant the ones stored in Documentation, which is what we considered the authoritative source and were planning on adopting everywhere we use device tree (ie, fixing the rest of our code as a result of the discussions we have submitting the bindings). I'd heard some people refer to these as Linux specific, but I'm glad they're not as it means we won't be pushing upstream on getting everyone to agree on one set of bindings.

Does this mean I can submit bindings for devices that don't have a Linux driver? How about devices where it doesn't even really make sense to ever have a Linux driver?