Re: [PATCH v4 1/2] dt-bindings: timer: thead,c900-aclint-mtimer: separate mtime and mtimecmp regs

[Conor Dooley <conor@kernel.org>]

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On Thu, Nov 30, 2023 at 04:51:32PM +0530, Anup Patel wrote:
> On Thu, Nov 30, 2023 at 3:27 PM Conor Dooley <conor@kernel.org> wrote:
> >
> > On Thu, Nov 30, 2023 at 03:01:24PM +0530, Anup Patel wrote:
> > > On Sat, Nov 18, 2023 at 12:39 PM Inochi Amaoto <inochiama@outlook.com> wrote:
> > > >
> > > > The timer registers of aclint don't follow the clint layout and can
> > > > be mapped on any different offset. As sg2042 uses separated timer
> > > > and mswi for its clint, it should follow the aclint spec and have
> > > > separated registers.
> > > >
> > > > The previous patch introduced a new type of T-HEAD aclint timer which
> > > > has clint timer layout. Although it has the clint timer layout, it
> > > > should follow the aclint spec and uses the separated mtime and mtimecmp
> > > > regs. So a ABI change is needed to make the timer fit the aclint spec.
> > > >
> > > > To make T-HEAD aclint timer more closer to the aclint spec, use
> > > > regs-names to represent the mtimecmp register, which can avoid hack
> > > > for unsupport mtime register of T-HEAD aclint timer.
> > > >
> > > > Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
> > > > Fixes: 4734449f7311 ("dt-bindings: timer: Add Sophgo sg2042 CLINT timer")
> > > > Link: https://lists.infradead.org/pipermail/opensbi/2023-October/005693.html
> > > > Link: https://github.com/riscv/riscv-aclint/blob/main/riscv-aclint.adoc
> > >
> > > The ratified Priv v1.12 specification defines platform specific M-mode timer
> > > registers without defining any layout of mtime and mtimecmp registers.
> > > (Refer, "3.2.1 Machine Timer Registers (mtime and mtimecmp)")
> > >
> > > The "thead,c900-aclint-mtimer" can be thought of as is one possible
> > > implementation of "riscv,mtimer" defined by the Priv v1.12 specificaiton.
> > >
> > > If it is not too late then I suggest making this binding into generic
> > > "riscv,mtimer" binding.
> >
> > We could definitely reorganise things, it's not too late for that as
> > implementation specific compatibles would be needed regardless, so
> > software that would've matched on those will continue to do so.
> >
> > That said, does this platform actually implement the 1.12 priv spec if
> > there is no mtime register? The section you reference says:
> > "Platforms provide a real-time counter, exposed as a memory-mapped
> > machine-mode read-write register, mtime." It seems to me like this
> > hardware is not suitable for a generic "riscv,mtimer" fallback.
> 
> Yes, the T-Head mtimer does not implement both mtime and mtimecmp
> so technically it only implements a portion of the ratified RISC-V mtimer
> chapter.
> 
> >
> > Am I missing something there Anup?
> >
> > It doesn't even implement the draft aclint spec, given that that says:
> > "The MTIMER device provides machine-level timer functionality for a set
> > of HARTs on a RISC-V platform. It has a single fixed-frequency monotonic
> > time counter (MTIME) register and a time compare register (MTIMECMP) for
> > each HART connected to the MTIMER device."
> >
> > But I already said no to having a generic, "riscv" prefixed, compatible
> > for that, given it is in draft form.
> 
> I am not suggesting T-Head timer implements aclint spec. Also,
> since aclint spec is in draft state it is out of the question.

I did not intend to imply that you were suggesting that there should be
one. I was just trying to clarify that I was not trying to bring back
the topic of a generic aclint binding applying here.

> My suggestion is to treat "3.2.1 Machine Timer Registers (mtime
> and mtimecmp)" as RISC-V mtimer defined by the RISC-V privileged
> specification and define "riscv" prefixed DT binding for this.

I'm not against a binding for that at all.

> This binding defines two possible values for "reg" property:
> 1) contains two items: a) mtime register address and,
>      b) base address of mtimecmp registers
> 2) contain one item: a) base address of mtimecmp registers
> 
> The t-head mtimer seems to implement #2 whereas the RISC-V
> mtimer (Priv spec) aligns with #1.
> 
> If we want to keep this DT binding t-head specific then
> we should remove option #1 (above) from this DT binding

This part is already the conclusion of one of the other "branches" of
this thread and is (AFAIU) Inochi's plan for the next version.

> and add separate "riscv" prefixed DT binding for RISC-V mtimer.

Do you know of any users for a "riscv,mtimer" binding that are not
covered by existing bindings for the clint?

Cheers,
Conor.

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