Re: [PATCH tty 1/1] serial: 8250: Change console_msr_work to IRQ_WORK_LAZY

Re: [PATCH tty 1/1] serial: 8250: Change console_msr_work to IRQ_WORK_LAZY

[Jon Hunter]

On 28/08/2026 17:06, Sebastian Andrzej Siewior wrote:
> On 2026-08-28 18:04:08 [+0206], John Ogness wrote:
>> For some platforms it is a problem to queue irq_work when entering
>> cpuidle states. Since irq_work is used for deferred MSR handling,
>> any atomic console printing when entering cpuidle states can lead
>> to the affected hardware hanging. Tegra20 and Tegra30 are examples
>> of such platforms. Avoiding raising the irq_work IRQ has shown to
>> circumvent the problem.
> 
> I would argue that this is done for the benefit of less interrupts since
> there no need to handle console_msr_work immediately. With the side
> effect that it might fix a bug that was not yet reported on Tegra[23]0.
> 
> Ideally the Tegra folks should figure out what exactly is broken instead
> of adding duct tape everywhere else.

Yes that would be ideal. Unfortunately these are old devices now and very
much in maintenance mode so getting to the bottom of this now is unlikely.
You may say then why don't we completely deprecate these, but then you
could argue the other way and say they have been working fine up until
now.

Jon

-- 
nvpublic

Re: [PATCH tty 1/1] serial: 8250: Change console_msr_work to IRQ_WORK_LAZY

[Andy Shevchenko]

On Fri, Aug 28, 2026 at 06:06:50PM +0100, Jon Hunter wrote:
> On 28/08/2026 17:06, Sebastian Andrzej Siewior wrote:
> > On 2026-08-28 18:04:08 [+0206], John Ogness wrote:
> > > For some platforms it is a problem to queue irq_work when entering
> > > cpuidle states. Since irq_work is used for deferred MSR handling,
> > > any atomic console printing when entering cpuidle states can lead
> > > to the affected hardware hanging. Tegra20 and Tegra30 are examples
> > > of such platforms. Avoiding raising the irq_work IRQ has shown to
> > > circumvent the problem.
> > 
> > I would argue that this is done for the benefit of less interrupts since
> > there no need to handle console_msr_work immediately. With the side
> > effect that it might fix a bug that was not yet reported on Tegra[23]0.
> > 
> > Ideally the Tegra folks should figure out what exactly is broken instead
> > of adding duct tape everywhere else.
> 
> Yes that would be ideal. Unfortunately these are old devices now and very
> much in maintenance mode so getting to the bottom of this now is unlikely.
> You may say then why don't we completely deprecate these, but then you
> could argue the other way and say they have been working fine up until
> now.

This is a trade-off between burden of supporting them vs. developing and move
on on more important things. Personally I consider non-blocking console feature
is *much* more important than a few "working fine up until now" outdated
platforms (side example, recently the whole i486 support was ripped off, while
working fine...).

-- 
With Best Regards,
Andy Shevchenko

Re: [PATCH tty 1/1] serial: 8250: Change console_msr_work to IRQ_WORK_LAZY

[Sebastian Andrzej Siewior]

On 2026-08-28 18:06:50 [+0100], Jon Hunter wrote:
> 
> On 28/08/2026 17:06, Sebastian Andrzej Siewior wrote:
> > On 2026-08-28 18:04:08 [+0206], John Ogness wrote:
> > > For some platforms it is a problem to queue irq_work when entering
> > > cpuidle states. Since irq_work is used for deferred MSR handling,
> > > any atomic console printing when entering cpuidle states can lead
> > > to the affected hardware hanging. Tegra20 and Tegra30 are examples
> > > of such platforms. Avoiding raising the irq_work IRQ has shown to
> > > circumvent the problem.
> > 
> > I would argue that this is done for the benefit of less interrupts since
> > there no need to handle console_msr_work immediately. With the side
> > effect that it might fix a bug that was not yet reported on Tegra[23]0.
> > 
> > Ideally the Tegra folks should figure out what exactly is broken instead
> > of adding duct tape everywhere else.
> 
> Yes that would be ideal. Unfortunately these are old devices now and very
> much in maintenance mode so getting to the bottom of this now is unlikely.

So this is maintenance mode.

> You may say then why don't we completely deprecate these, but then you
> could argue the other way and say they have been working fine up until
> now.

So a couple of years ago this platform stalled printk development due
its irq_work usage. Now again. Imagine we would have a ext4 feature
which freezes just this platform on boot. What would happen here?

The current workaround is to use LAZY which in general defers the IRQ
work to the next tick and it works for you. If you enable NOHZ and the
CPU goes tickless even a scheduled LAZY irq-work will raise the irq-work
interrupt to ensure the CPU does not go idle and we do have a wake up.

This might bring the problem back at some point.

> Jon

Sebastian

Re: [PATCH tty 1/1] serial: 8250: Change console_msr_work to IRQ_WORK_LAZY

[Jon Hunter]

On 31/08/2026 09:20, Sebastian Andrzej Siewior wrote:
> On 2026-08-28 18:06:50 [+0100], Jon Hunter wrote:
>>
>> On 28/08/2026 17:06, Sebastian Andrzej Siewior wrote:
>>> On 2026-08-28 18:04:08 [+0206], John Ogness wrote:
>>>> For some platforms it is a problem to queue irq_work when entering
>>>> cpuidle states. Since irq_work is used for deferred MSR handling,
>>>> any atomic console printing when entering cpuidle states can lead
>>>> to the affected hardware hanging. Tegra20 and Tegra30 are examples
>>>> of such platforms. Avoiding raising the irq_work IRQ has shown to
>>>> circumvent the problem.
>>>
>>> I would argue that this is done for the benefit of less interrupts since
>>> there no need to handle console_msr_work immediately. With the side
>>> effect that it might fix a bug that was not yet reported on Tegra[23]0.
>>>
>>> Ideally the Tegra folks should figure out what exactly is broken instead
>>> of adding duct tape everywhere else.
>>
>> Yes that would be ideal. Unfortunately these are old devices now and very
>> much in maintenance mode so getting to the bottom of this now is unlikely.
> 
> So this is maintenance mode.
> 
>> You may say then why don't we completely deprecate these, but then you
>> could argue the other way and say they have been working fine up until
>> now.
> 
> So a couple of years ago this platform stalled printk development due
> its irq_work usage. Now again. Imagine we would have a ext4 feature
> which freezes just this platform on boot. What would happen here?

I simply reported a regression. I did not set out to stall any work.
If the work stalled because of this issue alone, why was this not
discussed back when this happened? I am sure we could have come to
a reasonable comprise.

Please note that these legacy boards have caught some other issues in
the past and so I do see some value in keeping them working. This one
is a bit of an odd ball because clearly this platform has some issues
related to CPU idle. Again we are happy to discuss that and come to a
reasonable comprise.
> The current workaround is to use LAZY which in general defers the IRQ
> work to the next tick and it works for you. If you enable NOHZ and the
> CPU goes tickless even a scheduled LAZY irq-work will raise the irq-work
> interrupt to ensure the CPU does not go idle and we do have a wake up.
> 
> This might bring the problem back at some point.

That may be, and we will cross that bridge if and when we come to it.

Jon

-- 
nvpublic