linux-kernel - Re: [PATCH 02/26] serial: altera_jtaguart: drop low-latency workaround

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Message-ID: <20210421100352.vayt5himhl7php4h@distanz.ch>
Date:   Wed, 21 Apr 2021 12:03:52 +0200
From:   Tobias Klauser <tklauser@...tanz.ch>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 02/26] serial: altera_jtaguart: drop low-latency
 workaround

On 2021-04-21 at 11:54:45 +0200, Johan Hovold <johan@...nel.org> wrote:
> Commit 53dd0ba7a6f4 ("tty: serial: altera_jtag: drop uart_port->lock
> before calling tty_flip_buffer_push()") claimed to address a locking
> issue but only provided a dubious lockdep splat from an unrelated
> driver, which in the end turned out to be due a broken local change
> carried by the author.
> 
> Unfortunately these patches were merged before the issue had been
> analysed properly so the commit messages makes no sense whatsoever.
> 
> The real issue was first seen on RT which at the time effectively always
> set the low_latency flag for all serial drivers by patching
> tty_flip_buffer_push(). This in turn revealed that many drivers did not
> handle the infamous low_latency behaviour which meant that data was
> pushed immediately to the line discipline instead of being deferred to a
> work queue.
> 
> Since commit a9c3f68f3cd8 ("tty: Fix low_latency BUG"),
> tty_flip_buffer_push() always schedules a work item to push data to the
> line discipline and there's no need to keep any low_latency hacks
> around.
> 
> Link: https://lore.kernel.org/linux-serial/cover.1376923198.git.viresh.kumar@linaro.org/
> Cc: Tobias Klauser <tklauser@...tanz.ch>
> Signed-off-by: Johan Hovold <johan@...nel.org>

Acked-by: Tobias Klauser <tklauser@...tanz.ch>

Thanks