Re: [PATCH 1/3] tty: serial: 8250_omap: do not use RX DMA if pause is not supported

[Peter Hurley <peter@hurleysoftware.com>]

On 08/07/2015 04:00 PM, Sebastian Andrzej Siewior wrote:
> 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: <stable@vger.kernel.org>
> 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;

No offense, Sebastian, but it boggles my mind that anyone could defend this
as solid api design. We're in the middle of an interrupt handler and the
slave dma driver is /just/ telling us now that it doesn't implement this
functionality?!!?

The dmaengine api has _so much_ setup and none of it contemplates telling the
consumer that critical functionality is missing?

Even dma_get_slave_caps() returns _true_ for cmd_pause support; ok, that
interface is pointless.

Rather than losing /critical data/ here, the interrupt handler should just
busy-wait until dmaengine_tx_status() returns DMA_COMPLETE for the rx_cookie.

Regards,
Peter Hurley

>  	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)
>