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

Message ID 20210421095509.3024-6-johan@kernel.org
State New
Headers show
Series
  • tty: drop low-latency workarounds
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Commit Message

Johan Hovold April 21, 2021, 9:54 a.m.
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@linaro.org>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/tty/serial/amba-pl011.c | 2 --
 1 file changed, 2 deletions(-)

Comments

Linus Walleij April 22, 2021, 12:18 a.m. | #1
On Wed, Apr 21, 2021 at 11:55 AM Johan Hovold <johan@kernel.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@linaro.org>

> Cc: Russell King <linux@armlinux.org.uk>

> Signed-off-by: Johan Hovold <johan@kernel.org>


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


Yours,
Linus Walleij

Patch

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)