Regression: serial: imx: overrun errors on debug UART

[Uwe Kleine-König]

Hello Greg,

On Tue, Jun 20, 2023 at 04:59:18PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Jun 20, 2023 at 04:47:10PM +0200, Linux regression tracking (Thorsten Leemhuis) wrote:
> > On 24.05.23 15:07, Stefan Wahren wrote:
> > > 
> > > Am 23.05.23 um 21:44 schrieb Sergey Organov:
> > >> "Linux regression tracking (Thorsten Leemhuis)"
> > >> <regressions at leemhuis.info> writes:
> > >>
> > >> Solving this would need to identify the cause of interrupts being
> > >> disabled for prolonged times, and nobody volunteered to investigate this
> > >> further. One suspect, the Linux serial console, has been likely excluded
> > >> already though, as not actually being in use for printk() output.
> > >>
> > > 
> > > I don't think that we can exclude the serial console as a whole, i never
> > > made such a observation. But at least we can exclude kernel logging on
> > > the debug UART.
> > 
> > Stefan, just wondering: was this ever addressed upstream? I assume it's
> > not, just wanted to be sure.
> > 
> > I'm a bit unsure what to do with this and consider asking Greg for
> > advice, as he applied the patch. On one hand it's *IMHO* clearly a
> > regression (but for the record,  some people involved in the discussion
> > claim it's not). OTOH the culprit was applied more than a year ago now,
> > so reverting it might cause more trouble than it's worth at this point,
> > as that could lead to regressions for other users.
> 
> I'll be glad to revert this, but for some reason I thought that someone
> was working on a "real fix" here.  Stefan, is that not the case?

Sergey Organov already said something similar, but not very explicit:
With the current understanding reverting said commit is wrong. It is
expected that the commit increases irq latency for imx-serial a bit for
the benefit of less interrupts and so serves the overall system
performance. That this poses a problem only means that on the reporter's
machine there is already an issue that results in a longer period with
disabled irqs. While reverting the imx-serial commit would (maybe) solve
that, the actual problem is the other issue that disables preemption for
a longer timespan.

So TL;DR: Please don't revert the imx-serial patch.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |
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