Regression: serial: imx: overrun errors on debug UART

[Greg Kroah-Hartman]

On Tue, Jun 20, 2023 at 09:27:48PM +0200, Uwe Kleine-König wrote:
> 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.

Ok, will leave this alone, it shouldn't be marked as a regression.

greg k-h