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

From: Samuel Holland <samuel.holland@sifive.com>
To: Emil Renner Berthing <emil.renner.berthing@canonical.com>,
	Anup Patel <apatel@ventanamicro.com>,
	Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Daniel Lezcano <daniel.lezcano@linaro.org>
Subject: Re: [PATCH v1 0/9] Fix Allwinner D1 boot regression
Date: Thu, 15 Aug 2024 10:59:37 -0500	[thread overview]
Message-ID: <085c8332-d7f1-41df-8854-bee06291ba83@sifive.com> (raw)
In-Reply-To: <CAJM55Z9kKqs-kMubsGsRkS6E2Y4ur1MmwD+1XFvGP=UVNrJvRg@mail.gmail.com>

Hi Emil,

On 2024-08-15 10:07 AM, Emil Renner Berthing wrote:
> Samuel Holland wrote:
>> On 2024-08-15 9:16 AM, Anup Patel wrote:
>>> On Thu, Aug 15, 2024 at 7:41 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>>>>
>>>> On Thu, Aug 15 2024 at 08:32, Samuel Holland wrote:
>>>>> On 2024-08-15 8:16 AM, Thomas Gleixner 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.
>>>>
>>>> Sigh. Does RISCV really have to repeat all mistakes which have been made
>>>> by x86, ARM and others before? It's known for decades that the kernel
>>>> relies on a working timer...
>>>
>>> My apologies for the delay in finding a fix for this issue.
>>>
>>> Almost all RISC-V platforms (except this one) have SBI Timer always
>>> available and Linux uses a better timer or Sstc extension whenever
>>> it is available.
>>
>> So this is the immediate solution: add the CLINT to the firmware devicetree so
>> that the SBI time extension works, and Linux will boot without any code changes,
>> albeit with a higher-overhead clockevent device.
> 
> But this will mean that you can't update your kernel to v6.9 or newer without
> reflashing OpenSBI and u-boot. That's still a regression right?

I suppose that depends on if you think the SBI time extension is (or should have
been) mandatory for platforms without Sstc. If the SBI time extension is
mandatory, then this is a firmware bug, and not really Linux's responsibility to
work around.

If the SBI time extension is not mandatory, then Linux needs to be able to
handle platforms where the S-mode visible timer is attached to an external
interrupt controller (PLIC or APLIC), so the irqchip driver needs to be loaded
before time_init() (timer_probe()). So in that case, the bug is a Linux
regression, and we would need to revert the platform driver conversion.

Regards,
Samuel

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