linux-kernel - [PATCH 08/26] serial: arc_uart: drop low-latency workaround

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Message-Id: <20210421095509.3024-9-johan@kernel.org>
Date:   Wed, 21 Apr 2021 11:54:51 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jirislaby@...nel.org>, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, Johan Hovold <johan@...nel.org>,
        Vineet Gupta <vgupta@...opsys.com>
Subject: [PATCH 08/26] serial: arc_uart: drop low-latency workaround

Commit 3fa1200851c7 ("tty: serial: arc: 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: Vineet Gupta <vgupta@...opsys.com>
Signed-off-by: Johan Hovold <johan@...nel.org>
---
 drivers/tty/serial/arc_uart.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/tty/serial/arc_uart.c b/drivers/tty/serial/arc_uart.c
index 17c3fc398fc6..1a9444b6b57e 100644
--- a/drivers/tty/serial/arc_uart.c
+++ b/drivers/tty/serial/arc_uart.c
@@ -236,9 +236,7 @@ static void arc_serial_rx_chars(struct uart_port *port, unsigned int status)
 		if (!(uart_handle_sysrq_char(port, ch)))
 			uart_insert_char(port, status, RXOERR, ch, flg);

-		spin_unlock(&port->lock);
 		tty_flip_buffer_push(&port->state->port);
-		spin_lock(&port->lock);
 	} while (!((status = UART_GET_STATUS(port)) & RXEMPTY));
 }

-- 
2.26.3