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

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Date:   Mon, 17 Oct 2022 11:08:41 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     Matthias Schiffer <matthias.schiffer@...tq-group.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Tony Lindgren <tony@...mide.com>,
        Peter Hurley <peter@...leysoftware.com>,
        linux-serial <linux-serial@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] serial: 8250_omap: remove wait loop from Errata i202
 workaround

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")

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.

Either way I would like to see Tony's ACK before this is getting removed
as suggested in this patch.

> Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> 
> Thanks.

Sebastian