Re: [PATCH] serial: 8250_omap: remove wait loop from Errata i202 workaround

[Tony Lindgren <tony@atomide.com>]

Hi,

Adding Nishanth to Cc also.

* Ilpo Järvinen <ilpo.jarvinen@linux.intel.com> [221017 12:06]:
> On Mon, 17 Oct 2022, Sebastian Andrzej Siewior wrote:
> 
> > On 2022-10-17 11:12:41 [+0300], Ilpo Järvinen wrote:
> > > On Thu, 13 Oct 2022, Matthias Schiffer wrote:
> > > 
> > > > We were occasionally seeing the "Errata i202: timedout" on an AM335x
> > > > board when repeatedly opening and closing a UART connected to an active
> > > > sender. As new input may arrive at any time, it is possible to miss the
> > > > "RX FIFO empty" condition, forcing the loop to wait until it times out.
> > > 
> > > I can see this problem could occur and why your patch fixes it.
> > > 
> > > > Nothing in the i202 Advisory states that such a wait is even necessary;
> > > > other FIFO clear functions like serial8250_clear_fifos() do not wait
> > > > either. For this reason, it seems safe to remove the wait, fixing the
> > > > mentioned issue.
> > > 
> > > Checking the commit that added this driver and the loop along with it, 
> > > there was no information why it would be needed there either.
> > 
> > I don't remember all the details but I do remember that I never hit it.
> > The idea back then was to document what appears the problem and then
> > once there is a reproducer address it _or_ when there is another problem
> > check if it aligns with the output here (so that _this_ problem's origin
> > could be this). This was part of address all known chip erratas and
> > copied from omap-serial at the time so that the 8250 does not miss
> > anything.
> > Looking closer, this is still part of the omap-serial driver and it was
> > introduced in commit
> >    0003450964357 ("omap2/3/4: serial: errata i202: fix for MDR1 access")
> 
> I found that one too but it doesn't give any explanation for it either.
> In fact, the wait for empty is mysteriously missing from the itemized
> description of the workaround in the commit message.
> 
> > If someone found a way to trigger this output which is unrelated to the
> > expected cause then this is clearly not helping nor intended.
> > 
> > I would prefer to keep the loop and replace the disturbing output with a
> > comment describing _why_ the FIFO might remain non-empty after a flush.
> > 
> > In worst cases that loop causes a delay of less than 0.5ms while setting
> > a baud rate so I doubt that this is causing a real problem.

This sounds like a safe solution for me if it's needed.

> > Either way I would like to see Tony's ACK before this is getting removed
> > as suggested in this patch.
> 
> Thanks for chimming in.
> 
> I went to do some lore searching and came across this thread (it should 
> be added with Link: tag the patch regardless of its final form):
>   https://lore.kernel.org/linux-omap/4BBF61FE.3060807@ti.com/

Nishanth, do you have any more info on checking for fifo empty here?

Regards,

Tony