Re: [PATCH 1/3] tty: serial: 8250_omap: do not use RX DMA if pause is not supported
From: Peter Ujfalusi <hidden>
Date: 2015-08-10 11:54:33
Also in:
linux-omap, lkml
On 08/07/2015 11:00 PM, Sebastian Andrzej Siewior wrote:
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The 8250-omap driver requires the DMA-engine driver to support the pause command in order to properly turn off programmed RX transfer before the driver stars manually reading from the FIFO. The lacking support of the requirement has been discovered recently. In order to stay safe here we disable support for RX-DMA as soon as we notice that it does not work. This should happen very early. If the user does not want to see this backtrace he can either disable DMA support (completely or RX-only) or backport the required patches for edma / omap-dma once they hit mainline. Cc: <redacted> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- drivers/tty/serial/8250/8250_omap.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-)diff --git a/drivers/tty/serial/8250/8250_omap.c b/drivers/tty/serial/8250/8250_omap.c index 0340ee6ba970..07a11e0935e4 100644 --- a/drivers/tty/serial/8250/8250_omap.c +++ b/drivers/tty/serial/8250/8250_omap.c@@ -112,6 +112,7 @@ struct omap8250_priv { struct work_struct qos_work; struct uart_8250_dma omap8250_dma; spinlock_t rx_dma_lock; + bool rx_dma_broken; }; static u32 uart_read(struct uart_8250_port *up, u32 reg)@@ -761,6 +762,7 @@ static void omap_8250_rx_dma_flush(struct uart_8250_port *p) struct omap8250_priv *priv = p->port.private_data; struct uart_8250_dma *dma = p->dma; unsigned long flags; + int ret; spin_lock_irqsave(&priv->rx_dma_lock, flags);@@ -769,7 +771,9 @@ static void omap_8250_rx_dma_flush(struct uart_8250_port *p) return; } - dmaengine_pause(dma->rxchan); + ret = dmaengine_pause(dma->rxchan); + if (WARN_ON_ONCE(ret)) + priv->rx_dma_broken = true;
I don't think this is good thing for the stable _and_ for the mainline at the same time: in stable the rx DMA should not be allowed since the stable kernels does not allow pause/resume with omap-dma, so there the rx DMA should be just disabled for UART. This change will cause regression since it introduce a WARN_ON_ONCE, which will be printed if the user tries to use non working feature. In mainline you will eventually going to have pause/resume support so this patch will make no sense there.
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spin_unlock_irqrestore(&priv->rx_dma_lock, flags);@@ -813,6 +817,9 @@ static int omap_8250_rx_dma(struct uart_8250_port *p, unsigned int iir) break; } + if (priv->rx_dma_broken) + return -EINVAL; + spin_lock_irqsave(&priv->rx_dma_lock, flags); if (dma->rx_running)
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