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

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Date:   Wed, 21 Apr 2021 11:54:48 +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>,
        Linus Walleij <linus.walleij@...aro.org>,
        Russell King <linux@...linux.org.uk>
Subject: [PATCH 05/26] serial: amba-pl011: drop low-latency workaround

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>
---
 drivers/tty/serial/amba-pl011.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 4ead0c9048a8..78682c12156a 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -937,12 +937,10 @@ static void pl011_dma_rx_chars(struct uart_amba_port *uap,
 		fifotaken = pl011_fifo_to_tty(uap);
 	}

-	spin_unlock(&uap->port.lock);
 	dev_vdbg(uap->port.dev,
 		 "Took %d chars from DMA buffer and %d chars from the FIFO\n",
 		 dma_count, fifotaken);
 	tty_flip_buffer_push(port);
-	spin_lock(&uap->port.lock);
 }

 static void pl011_dma_rx_irq(struct uart_amba_port *uap)
-- 
2.26.3