linux-kernel - Re: [PATCH 05/26] serial: amba-pl011: drop low-latency workaround

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Date:   Thu, 22 Apr 2021 02:18:18 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH 05/26] serial: amba-pl011: drop low-latency workaround

On Wed, Apr 21, 2021 at 11:55 AM Johan Hovold <johan@...nel.org> wrote:

> Commit ead76f329f77 ("ARM: 6763/1: pl011: add optional RX DMA to PL011
> v2") added RX DMA support and also reproduced the workaround for the
> infamous low_latency behaviour of tty_flip_buffer_push() by dropping and
> reacquiring the port lock during receive processing.
>
> 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.
>
> Note that the port lock is also dropped in the PIO path
> (see pl011_rx_chars), but it is not clear whether this is still needed
> by the DMA code added by the aforementioned commit.
>
> Cc: Linus Walleij <linus.walleij@...aro.org>
> Cc: Russell King <linux@...linux.org.uk>
> Signed-off-by: Johan Hovold <johan@...nel.org>

Looks like the right thing to do to me! Thanks for digging this out.
Reviewed-by: Linus Walleij <linus.walleij@...aro.org>

Yours,
Linus Walleij