Message-ID: <74d255be524594806b6f9a2cd8bb6d2ab4915340.camel@icenowy.me>
Date: Fri, 16 Aug 2024 14:13:22 +0800
From: Icenowy Zheng <uwu@...nowy.me>
To: Palmer Dabbelt <palmer@...belt.com>, samuel.holland@...ive.com
Cc: Renner Berthing <emil.renner.berthing@...onical.com>, 
	apatel@...tanamicro.com, tglx@...utronix.de, linux-kernel@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, Paul Walmsley <paul.walmsley@...ive.com>, 
	aou@...s.berkeley.edu, daniel.lezcano@...aro.org
Subject: Re: [PATCH v1 0/9] Fix Allwinner D1 boot regression

在 2024-08-15星期四的 10:51 -0700，Palmer Dabbelt写道：
> On Thu, 15 Aug 2024 08:59:37 PDT (-0700),
> samuel.holland@...ive.com wrote:
> > 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@...utronix.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...
> 
> It's even worse than that: RISC-V doesn't even mandate any working 
> _instructions_, much less anything in the platform/firmware.
> 
> > > > > 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?
> 
> Ya, I'd call that a regression.  Updating the firmware on these
> things 
> isn't generally something we can rely on users to do, we've worked 
> around other firmware bugs where we can to avoid forced updates.
> 
> > 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.
> 
> It doesn't really matter what the specs say (aka intended to say in 
> RISC-V land): if there's a regression then we have to deal with it.  
> It's not like whatever's written in the specs actually matters,
> vendors 
> can just do whatever they want, so wer'e just stuck making the known 
> implementations work.
> 
> So I think if the revert is the best fix then we should revert it.
> 
> That said: If the CLINT works, could we just add a probing quirk to
> make 

CLINT works, but will will never work in S mode by its design -- the
register used is all M-mode-only.

So it this kind of probing quirk is being added, the target will be
OpenSBI instead of Linux, and the problem of updating firmware still
exists.

> it appear on these systems even when it's not in the DT?  I'm
> thinking 
> something like adding a compatibly string to the CLINT driver for the
> SOC (or core or whatever, just something that's already there).  We'd
> probably need a bit of special-case probing code, but shouldn't be so
> bad.  We've got some other compatibility-oriented DT quirks floating 
> around.
> 
> > Regards,
> > Samuel
> 
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