Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756420AbdDGJqO (ORCPT ); Fri, 7 Apr 2017 05:46:14 -0400 Received: from bhuna.collabora.co.uk ([46.235.227.227]:56569 "EHLO bhuna.collabora.co.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755381AbdDGJqH (ORCPT ); Fri, 7 Apr 2017 05:46:07 -0400 From: Peter Senna Tschudin Cc: Peter Senna Tschudin , Greg Kroah-Hartman , Jiri Slaby , linux-serial@vger.kernel.org (open list:SERIAL DRIVERS), linux-kernel@vger.kernel.org (open list) Subject: [PATCH] imx-serial: Reduce RX DMA startup latency when opening for reading Date: Fri, 7 Apr 2017 11:45:24 +0200 Message-Id: <20170407094524.26390-1-peter.senna@collabora.com> X-Mailer: git-send-email 2.9.3 To: unlisted-recipients:; (no To-header on input) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4698 Lines: 152 Reduce RX DMA start latency for the first reception when port is opened for reading. Instead of waiting for an interrupt signaling data on RX FIFO or data too old on RX FIFO, start RX DMA immediately when the serial port is opened for reading. Before this patch, the average RX DMA latency for the first reception was 42489 microseconds with a standard deviation of 25721 microseconds in 36 samples. After the patch the average RX DMA latency for the first reception, when the serial port is opened for reading, is 653 microseconds with a standard deviation of 294 microseconds in 36 samples. Signed-off-by: Peter Senna Tschudin --- RX DMA latency for the first reception was calculated as the time difference between the call to imx_startup() and the completion of start_rx_dma(). I used this patch for the measurement: --- a/drivers/tty/serial/imx.c +++ b/drivers/tty/serial/imx.c @@ -238,6 +238,7 @@ struct imx_port { wait_queue_head_t dma_wait; unsigned int saved_reg[10]; bool context_saved; + struct timespec ts; }; struct imx_port_ucrs { @@ -1059,6 +1060,7 @@ static int start_rx_dma(struct imx_port *sport) struct dma_chan *chan = sport->dma_chan_rx; struct device *dev = sport->port.dev; struct dma_async_tx_descriptor *desc; + struct timespec te; int ret; sport->rx_ring.head = 0; @@ -1087,6 +1089,10 @@ static int start_rx_dma(struct imx_port *sport) sport->dma_is_rxing = 1; sport->rx_cookie = dmaengine_submit(desc); dma_async_issue_pending(chan); + + getnstimeofday(&te); + dev_err(sport->port.dev, "DMA delay(microseconds): %lu\n", (te.tv_nsec - sport->ts.tv_nsec)/1000); + return 0; } @@ -1286,6 +1292,8 @@ static int imx_startup(struct uart_port *port) int retval, i, readcnt = 0; unsigned long flags, temp; + getnstimeofday(&sport->ts); + retval = clk_prepare_enable(sport->clk_per); if (retval) return retval; drivers/tty/serial/imx.c | 63 +++++++++++++++++++++++++++++++++++++----------- 1 file changed, 49 insertions(+), 14 deletions(-) diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c index e3e152c..b4340b5 100644 --- a/drivers/tty/serial/imx.c +++ b/drivers/tty/serial/imx.c @@ -719,6 +719,27 @@ static irqreturn_t imx_rxint(int irq, void *dev_id) return IRQ_HANDLED; } +static void imx_disable_rx_int(struct imx_port *sport) +{ + unsigned long temp; + + sport->dma_is_rxing = 1; + + /* disable the receiver ready and aging timer interrupts */ + temp = readl(sport->port.membase + UCR1); + temp &= ~(UCR1_RRDYEN); + writel(temp, sport->port.membase + UCR1); + + temp = readl(sport->port.membase + UCR2); + temp &= ~(UCR2_ATEN); + writel(temp, sport->port.membase + UCR2); + + /* disable the rx errors interrupts */ + temp = readl(sport->port.membase + UCR4); + temp &= ~UCR4_OREN; + writel(temp, sport->port.membase + UCR4); +} + static void clear_rx_errors(struct imx_port *sport); static int start_rx_dma(struct imx_port *sport); /* @@ -734,21 +755,8 @@ static void imx_dma_rxint(struct imx_port *sport) temp = readl(sport->port.membase + USR2); if ((temp & USR2_RDR) && !sport->dma_is_rxing) { - sport->dma_is_rxing = 1; - /* disable the receiver ready and aging timer interrupts */ - temp = readl(sport->port.membase + UCR1); - temp &= ~(UCR1_RRDYEN); - writel(temp, sport->port.membase + UCR1); - - temp = readl(sport->port.membase + UCR2); - temp &= ~(UCR2_ATEN); - writel(temp, sport->port.membase + UCR2); - - /* disable the rx errors interrupts */ - temp = readl(sport->port.membase + UCR4); - temp &= ~UCR4_OREN; - writel(temp, sport->port.membase + UCR4); + imx_disable_rx_int(sport); /* tell the DMA to receive the data. */ start_rx_dma(sport); @@ -1339,6 +1347,33 @@ static int imx_startup(struct uart_port *port) * Enable modem status interrupts */ imx_enable_ms(&sport->port); + + /* + * If the serial port is opened for reading start RX DMA immediately + * instead of waiting for RX FIFO interrupts. In our iMX53 the average + * delay for the first reception dropped from approximately 35000 + * microseconds to 1000 microseconds. + */ + if (sport->dma_is_enabled) { + struct tty_struct *tty = sport->port.state->port.tty; + struct tty_file_private *file_priv; + int readcnt = 0; + + spin_lock(&tty->files_lock); + + if (!list_empty(&tty->tty_files)) + list_for_each_entry(file_priv, &tty->tty_files, list) + if (!(file_priv->file->f_flags & O_WRONLY)) + readcnt++; + + spin_unlock(&tty->files_lock); + + if (readcnt > 0) { + imx_disable_rx_int(sport); + start_rx_dma(sport); + } + } + spin_unlock_irqrestore(&sport->port.lock, flags); return 0; -- 2.9.3