linux-kernel - Re: [PATCH] Revert "serial: 8250: Fix clearing FIFOs in RS485 mode again"

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Date:   Sun, 16 Dec 2018 23:01:18 +0100
From:   Marek Vasut <marex@...x.de>
To:     Paul Burton <paul.burton@...s.com>
Cc:     "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Paul Burton <pburton@...ecomp.com>,
        Daniel Jedrychowski <avistel@...il.com>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] Revert "serial: 8250: Fix clearing FIFOs in RS485 mode
 again"

On 12/16/2018 10:39 PM, Paul Burton wrote:
> Hi Marek,

Hi,

> On Sun, Dec 16, 2018 at 10:08:48PM +0100, Marek Vasut wrote:
>>> I did suggest an alternative approach which would rename
>>> serial8250_clear_fifos() and split it into 2 variants - one that
>>> disables FIFOs & one that does not, then use the latter in
>>> __do_stop_tx_rs485():
>>>
>>> https://lore.kernel.org/lkml/20181213014805.77u5dzydo23cm6fq@pburton-laptop/
>>>
>>> However I have no access to the OMAP3 hardware that Marek's patch was
>>> attempting to fix & have heard nothing back with regards to him testing
>>> that approach, so here's a simple revert that fixes the Ingenic JZ4780.
>>>
>>> I've marked for stable back to v4.10 presuming that this is how far the
>>> broken patch may be backported, given that this is where commit
>>> 2bed8a8e7072 ("Clearing FIFOs in RS485 emulation mode causes subsequent
>>> transmits to break") that it tried to fix was introduced.
>>
>> OK, I tested this on AM335x / OMAP3 and the system is again broken, so
>> that's a NAK.
> 
> To be clear - what did you test? This revert or the patch linked to
> above?
> 
> This revert would of course reintroduce your RS485 issue because it just
> undoes your change.

The revert. Which of the two patches do you need me to test.

> Either way, commit f6aa5beb45be ("serial: 8250: Fix clearing FIFOs in
> RS485 mode again") breaks systems that worked before it so at this late
> stage in the 4.20 cycle a revert would make sense to me. If that breaks
> RS85 on OMAP3 then my question would be how much can anyone really care
> if nobody noticed since v4.10? And why should that lead to you breaking
> the JZ4780 which has been discovered before a stable kernel release
> includes the breakage?

There's always a .y release where this can be properly investigated and
solved, instead of breaking one platform or the other.

Then again, see the patch from Ezequiel that was just posted, I think it
might be a far better solution.

-- 
Best regards,
Marek Vasut