linux-kernel - Re: [PATCH v1 0/9] Fix Allwinner D1 boot regression

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Message-ID: <CAK9=C2W1dNx5D9dUCDA1-iTGbPQz_utEjzcmOOaoPCHw_sSPpA@mail.gmail.com>
Date: Thu, 15 Aug 2024 21:23:23 +0530
From: Anup Patel <apatel@...tanamicro.com>
To: Samuel Holland <samuel.holland@...ive.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, 
	Emil Renner Berthing <emil.renner.berthing@...onical.com>, linux-kernel@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v1 0/9] Fix Allwinner D1 boot regression

On Thu, Aug 15, 2024 at 8:33 PM Samuel Holland
<samuel.holland@...ive.com> wrote:
>
> Hi Anup,
>
> On 2024-08-15 9:30 AM, Anup Patel wrote:
> > On Thu, Aug 15, 2024 at 7:02 PM Samuel Holland
> > <samuel.holland@...ive.com> wrote:
> >>> Yes. So the riscv timer is not working on this thing or it stops
> >>> somehow.
> >>
> >> That's correct. With the (firmware) devicetree that Emil is using, the OpenSBI
> >> firmware does not have a timer device, so it does not expose the (optional[1])
> >> SBI time extension, and sbi_set_timer() does nothing.
> >
> > OpenSBI uses platform specific M-mode timer (mtime and mtimecmp) to
> > provide SBI time extension to Linux.
> >
> > The RISC-V privileged specification (v1.10 or higher) requires platform to
> > provide a M-mode timer (mtime and mtimecmp).
> >
> > This platform not having any M-mode timer is yet another RISC-V spec
> > violation by this platform.
>
> You've misunderstood here. Allwinner D1 (T-HEAD C906) _does_ have an M-mode
> timer (a CLINT). It is just omitted from devicetree that Emil happens to be
> using, so OpenSBI isn't using it.
>
> Currently OpenSBI allows the system to boot without a timer device, and the SBI
> specification does not mandate the time extension. If consensus is that either
> of these should change, that's fine, but currently I see nothing in either the
> privileged spec nor the SBI spec that guarantees the availability of some timer
> to the kernel in S-mode.
>

The SBI time is certainly optional hence OpenSBI does not hang or crash if
it can't provide SBI time to supervisor software.

My comment is from the RISC-V privileged spec perspective:

1) Priv v1.10 says "Platforms provide a real-time counter, exposed as a
memory-mapped machine-mode register, mtime." in section "3.1.15 Machine
Timer Registers (mtime and mtimecmp)".

2) Similar statement in Priv v1.11 section "3.1.10 Machine Timer Registers
(mtime and mtimecmp)"

3) Similar statement in Priv v1.12 section "3.2.1 Machine Timer Registers
(mtime and mtimecmp)"

But since the M-mode timer was omitted from the DT, I think the DT was
always incomplete from the M-mode perspective.

Regards,
Anup