Re: [PATCH v4 1/2] riscv-platform-spec: PLIC and CLINT for Linux-2022 platform


Abner Chang
 



Anup Patel <Anup.Patel@...> 於 2021年5月25日 週二 下午12:39寫道:
Hi Alistar,

> -----Original Message-----
> From: tech-unixplatformspec@... <tech-
> unixplatformspec@...> On Behalf Of Alistair Francis
> Sent: 24 May 2021 12:54
> To: renba.chang@...
> Cc: sunilvl@...; tech-unixplatformspec@...;
> mchitale@...
> Subject: Re: [RISC-V] [tech-unixplatformspec] [PATCH v4 1/2] riscv-platform-
> spec: PLIC and CLINT for Linux-2022 platform
>
> On Mon, 2021-05-24 at 14:52 +0800, Abner Chang wrote:
> >
> >
> > Alistair Francis <Alistair.Francis@...> 於 2021年5月24日 週一
> > 上午11:42寫道:
> > > On Mon, 2021-05-24 at 11:03 +0800, Abner Chang wrote:
> > > > From: Abner Chang <renba.chang@...>
> > > >
> > > > Initial description of PLIC  CLINT section of Linux-2022 platform.
> > > >
> > > > Is this what we want to see of CLINT/Machine mode timer in the
> > > > platform spec?
> > > >
> > > > On v4 commit,
> > > > - PLIC section with [DEPRECATED] in Linux- 2022 chapter
> > > > - CLINT section in Linux- 2022 chapter for M-mode timer. We don't
> > > > mention
> > > >   IPI because AIA already supported it.
> > > > - In Embedded-2022 Machine mode timer section, CLINT is not
> > > > mandatory.
> > > > - Separate section in appendix for the Machine mode timer
> > > > registers
> > > >
> > > > On v3 commit,
> > > > - Address review comments.
> > > >
> > > > On v2 commit,
> > > > - CLINT is not deprecated.
> > > >
> > > > - Add a standalone section for Machine Mode Timer in System
> > > > Peripherals.
> > > >   Do you think this is a good place for Machine Mode Timer?
> > > >   @Mayuresh, please check if you are ok with this change, not sure
> > > if
> > > > this
> > > >   overlaps with your text or not (The timer setion). I can remove
> > > > this
> > > >   if you prefer to put this with your patch.
> > > >
> > > > - In Embedded-2022, refer to Machine Mode Timer in System
> > > Peripherals
> > > >   section and CLINT in Linux-2022 Platform.
> > > >   @Alistair, is this ok?
> > > >
> > > > On v1 commit,
> > > > - Not sure where to put the [DEPRECATED].
> > > > - Change the reference of PLIC in section 2.2.2. Interrupt
> > > Controller
> > > > to
> > > >   1.1.3.2 PLIC + CLINT section.
> > > >
> > > > Signed-off-by: Abner Chang <renba.chang@...>
> > > > Cc: Alistair Francis <alistair.francis@...>
> > > > Cc: Sunil V L <sunilvl@...>
> > > > Cc: Mayuresh Chitale <mchitale@...>
> > > > ---
> > > >  riscv-platform-spec.adoc | 104 +++++++++++++++++++++++----------
> > > > --
> > > --
> > > > --
> > > >  1 file changed, 62 insertions(+), 42 deletions(-)
> > > >
> > > > diff --git a/riscv-platform-spec.adoc b/riscv-platform-spec.adoc
> > > > index 160c74a..f8838ab 100644
> > > > --- a/riscv-platform-spec.adoc
> > > > +++ b/riscv-platform-spec.adoc
> > > > @@ -49,15 +49,37 @@ include::profiles.adoc[]
> > > >  * Start Address
> > > >
> > > >  ==== Interrupt Controller
> > > > -* AIA
> > > > -* PLIC + CLINT
> > > > -* Interrupt Assignments
> > > > +===== AIA[[AIA]]
> > > > +===== PLIC[DEPRECATED][[PLIC]]
> > > > +The Platform Level Interrupt Controller (PLIC) provides
> > > > facilities
> > > > to route
> > > > +the non-local interrupts to the external interrupt of a hart
> > > context
> > > > +with a given privilege mode in a given hart. The number of non-
> > > local
> > > > interrupt
> > > > +sources supported by PLIC and how does each of them connect to
> > > > the
> > > > hart
> > > > +context is PLIC core implementation-specific. + (Refer to
> > > >
> > > https://github.com/riscv/riscv-plic-spec/blob/master/riscv-plic.adoc
> > > [RISC-V
> > > >  PLIC Specification]
> > > > +for the implementation reference of PLIC operation parameters)
> > > > +
> > > > +===== CLINT[[CLINT]]
> > > > +On the contrast to PLIC, the Core Local Interrupt (CLINT)
> > > > provides
> > >
> > > I don't think this is a contrast to the PLIC, it's just a different
> > > functionality.
> > >
> >
> > Can rephase it.
> > >
> > > I'm also not sure the CLINT should be in the interrupt controller
> > > section. It is the Core Local INTerruptor (CLINT), not an interrupt
> > > controller.
> > >
> >
> > Hmm.. CLINT is not a sort of interrupt controller but seems to me it
> > is unnecessary to create another section apart from the Interrupt
> > Controller section.
>
> It probably makes more sense to add a timer section and put the CLNIT there
> instead of forcing it into the interrupt controller section, when it isn't really an
> interrupt controller.
>
> > >
> > > > +facilities to trigger local interrupt of
> > > <<MachineModeTimer,Machine
> > > > mode timer>> to hart.
> > > > +
> > > > +===== Interrupt Assignments
> > > >
> > > >  ==== System Peripherals
> > > > -* UART/Serial Console
> > > > -* Clocks
> > > > -* Timers
> > > > -* Watchdog Timers
> > > > +===== UART/Serial Console
> > > > +===== Clocks
> > > > +===== Timers
> > > > +
> > > > +===== Machine Mode Timer[[MachineModeTimer]] Machine mode
> timer
> > > > +is required for Linux-2022 platform and
> > > > incorporate with
> > > > +<<CLINT,CLINT>> to trigger the timer event to hart. The format
> > > > of
> > > > Machine mode timer operation parameters
> > > > +(`mtime` and `mtimecmp` registers) must compliant with RISC-V
> > >
> > > s/must compliant/must be compliant/g
> > >
> >
> > will correct this
> > >
> > > > Privilege specification section 3.1.10.
> > > > +The base address of the memory map registers of Machine mode
> > > > timer
> > > > is platform implementation-specific,
> > > > +however the offset of `mtime` and `mtimecmp` registers are fixed
> > > as
> > > > +<<machineModeTimerRegs,Machine mode timer registers>>.
> > > > +
> > > > +
> > > > +===== Watchdog Timers
> > > >
> > > >  ==== Boot Process
> > > >  * Firmware
> > > > @@ -289,9 +311,8 @@ Any RISC-V system that uses at least RV32/64G
> > > can
> > > > meet the Embedded-2022
> > > >  specification.
> > > >
> > > >  ==== Interrupt Controller
> > > > -Embedded systems are recommended to use a spec compliant
> > > > -https://github.com/riscv/riscv-plic-spec[PLIC], a spec compliant
> > > > -
> > > >
> > > https://github.com/riscv/riscv-fast-interrupt/blob/master/clic.adoc[
> > > CLIC]
> > > > +Embedded systems are recommended to use a spec compliant
> > > > <<PLIC,PLIC>>, a spec
> > > > +compliant
> > > >
> > > https://github.com/riscv/riscv-fast-interrupt/blob/master/clic.adoc[
> > > CLIC]
> > > >  or both a CLIC and and PLIC.
> > > >
> > > >  If using just a PLIC the system must continue to use the original
> > > > basic @@ -303,38 +324,9 @@ must be supported.
> > > >  Embedded systems cannot use a non-compliant interrupt controller
> > > and
> > > > still
> > > >  call it a PLIC or CLIC.
> > > >
> > > > -==== Machine Timer
> > > > -The RISC-V machine timer (controlled via `mtime` and `mtimecmp`)
> > > > must be -implemented. The two registers must be memory mapped as
> > > > required
> > > by
> > > > the RISC-V
> > > > -specification.
> > > > -
> > > > -The Embedded-2022 specification requires that the registers be
> > > > mapped -adjacent to each other with the `mtime` region at the
> > > > lower
> > > address.
> > > > -
> > > > -The starting address of this region can be located anywhere in
> > > > -memory, including inside other peripherals, as long as the start
> > > > address is
> > > > -4 byte aligned.
> > > > -
> > > > -An example of the memory layout for a 32-bit system with a single
> > > > hart is below
> > > > -
> > > > --------------------------
> > > > -=========================
> > > > -| 0x00 |  mtime low     |
> > > > -| 0x04 |  mtime high    |
> > > > -| 0x08 |  mtimecmp low  |
> > > > -| 0x0C |  mtimecmp high |
> > > > -=========================
> > > > --------------------------
> > > > -
> > > > -and for a 64-bit system with 2 harts
> > > > -
> > > > ----------------------------
> > > > -===========================
> > > > -| 0x00 |  mtime           |
> > > > -| 0x08 |  mtimecmp hart 1 |
> > > > -| 0x10 |  mtimecmp hart 2 |
> > > > -===========================
> > > > ----------------------------
> > > > +==== Machine Mode Timer
> > > > +The Embedded-2022 specification requires RISC-V
> > > > <<MachineModeTimer,Machine mode timer>> to be implemented.
> > > > +The Machine mode timer could be incorporated with the Core Local
> > > > Interrupt (<<CLINT, CLINT>>) or the specific event trigger
> > > mechanism
> > >
> > > CLINT is INTerruptor not just Interrupt.
> > >
> >
> > yes, will correct it
> > >
> > > > according to the platform design.
> > >
> > > What specific trigger mechanism?
> > >
> >
> > platform implementation-specific trigger mechanism?  or just remove
>
> I meant you didn't say what the mechanism is. It should be explicit, so that
> there are no ambiguities.
>
> > this? I put this just because the previous feedback says a small
> > system may not have CLINT for the m-mode timer.
>
> I would be in favour of saying either do what we already have written here OR
> a CLINT.
>
> If we go with something else that's ok, we just need to be clear about what it
> is.
>
> > >
> > > >
> > > >  ==== Memory Map
> > > >  It is recommended that main memory and loadable code (not ROM)
> > > start
> > > > at
> > > > @@ -349,5 +341,33 @@ When PMP is supported it is recommended to
> > > > include at least 4 regions, although
> > > >  if possible more should be supported to allow more flexibility.
> > > > Hardware
> > > >  implementations should aim for supporting at least 16 PMP
> > > > regions.
> > > >
> > > > +// Appendix sections
> > > > +== Appendix
> > > > +=== Machine Mode Timer Registers [[MachineModeTimerRegs]] The
> > > > +base address of Machine mode timer operation parameters is
> > > > platform implementation-specific. The Machine mode timer compare
> > > > registers (`mtimecmp`) for each hart is located at offset 0 and
> > > > the layout is organized as below table.
> > >
> > > Can we maintain what we have for the embedded spec?
> > >
> >
> > Do you mean to maintain the register layout(offset) originally defined
> > in the 2.2.3. for embedded systems?
>
> Yes. I worry that mandating a CLINT layout is too restrictive and that no one
> will end up following the embedded-2022 spec then. I would be in favour of
> saying CLINT or the memory layout we already have.

The ACLINT spec does not enforce any memory layout.
https://github.com/riscv/riscv-aclint/blob/master/riscv-aclint.adoc

The ACLINT is backward compatible to CLINT we should refer ACLINT
in-place of CLINT. This gives platform vendors flexibility to choose
their or memory layout and implement only certain parts of ACLINT
as required.

Hi Anup and Alistair. 
With ACLINT spec defined, I think we can revise this patch as below,
1.Remove CLINT from platform spec
2 Mention ACLINT in platform spec for Linux2020 platform and have a link to https://github.com/riscv/riscv-aclint/blob/master/riscv-aclint.adoc.
3. Remove Machine mode timer from system peripheral because that is in the scope of ACLINT
4. For Embedded-2022 platform, mention Machine mode timer and refer to ACLINT for the definition of registers.
Any other thoughts?

Abner


Regards,
Anup

>
> > I think we have the register layout as below to be compliant with the
> > m-mode timer registers defined in SiFive CLINT.
>
> I don't think that's flexible enough and that vendors won't want to follow it as
> they already have other IP they are using for timers.
>
> Alistair
>
> > Or do you mean something else?
> >
> > >  It allows the
> > > memory maps to be inside of a MMIO device. For example they can be
> > > registers inside of a general system controller.
> > >
> > >
> > > Also it looks like the lines are a little long here, can be bring
> > > them back to 80 charecters?
> > >
> >
> > sure.
> > >
> > > > +
> > > > +.Registers layout of mtimecmp
> > > > +[width="60%",cols="1,^3,^3"]
> > > > +|=======
> > > > +|*Offset*|*Register (8-byte) for RV64*|*Register (4-byte) for
> > > RV32*
> > > > +|`0x0000` |mtimecmp for hart 0 |mtimecmp low for hart 0
> > > > +|`0x0004` ||mtimecmp high for hart 0
> > > > +|`0x0008` |mtimecmp for hart 1 |mtimecmp low for hart 1
> > > > +|`0x000c` ||mtimecmp high for hart 1
> > > > +|... ||
> > > > +|`0x7ff0` |mtimecmp for hart 4094|mtimecmp low for hart 4094
> > > > +|`0x7ff4` ||mtimecmp high for hart 4094
> > > > +|=======
> > > > +
> > > > +The Machine mode timer counter (`mtime`) is located at offset
> > > 0x7ff8
> > > > from the base address of operation parameters.
> > > > +
> > > > +.Registers layout of mtime
> > > > +[width="60%",cols="1,^3,^3"]
> > > > +|=======
> > > > +|*Offset*|*Register (8-byte) for RV64*|*Register (4-byte) for
> > > RV32*
> > > > +|`0x7ff8` |mtime|mtime low
> > > > +|`0x7ffc` ||mtime high
> > > > +|=======
> > >
> > > Are you saying for a small embedded single core CPU we need to put
> > > mtimecmp at offset 0x00 and mtime at 0x7ff8? What about all the
> > > addresses between them?
> > >
> > > Alistair
> > >
> > > > +
> > > >  // acknowledge all of the contributors
> > > >  include::contributors.adoc[]
> > >
>
>
>
>
>

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