linux-kernel - Re: [PATCH 1/1] serial: 8250_dma: Rearm DMA Rx if more data is pending

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Date:   Mon, 7 Nov 2022 13:31:52 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
        Gilles BULOZ <gilles.buloz@...tron.com>
Subject: Re: [PATCH 1/1] serial: 8250_dma: Rearm DMA Rx if more data is
 pending

On Mon, Nov 07, 2022 at 12:21:26PM +0200, Ilpo Järvinen wrote:
> When DMA Rx completes, the current behavior is to just exit the DMA
> completion handler without future actions. If the transfer is still
> on-going, UART will trigger an interrupt and that eventually rearms the
> DMA Rx. The extra interrupt round-trip has an inherent latency cost
> that increases the risk of FIFO overrun. In such situations, the
> latency margin tends to already be less due to FIFO not being empty.
> 
> Add check into DMA Rx completion handler to detect if LSR has DR (Data
> Ready) still set. DR indicates there will be more characters pending
> and DMA Rx can be rearmed right away to handle them.

Yep, I used to have something like draft of the below change locally.
Thanks for putting it in shape and upstreamimg!

Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>

> Cc: Gilles BULOZ <gilles.buloz@...tron.com>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> ---
>  drivers/tty/serial/8250/8250_dma.c | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/8250/8250_dma.c b/drivers/tty/serial/8250/8250_dma.c
> index b85c82616e8c..37d6af2ec427 100644
> --- a/drivers/tty/serial/8250/8250_dma.c
> +++ b/drivers/tty/serial/8250/8250_dma.c
> @@ -38,9 +38,8 @@ static void __dma_tx_complete(void *param)
>  	spin_unlock_irqrestore(&p->port.lock, flags);
>  }
>  
> -static void __dma_rx_complete(void *param)
> +static void __dma_rx_complete(struct uart_8250_port *p)
>  {
> -	struct uart_8250_port	*p = param;
>  	struct uart_8250_dma	*dma = p->dma;
>  	struct tty_port		*tty_port = &p->port.state->port;
>  	struct dma_tx_state	state;
> @@ -57,6 +56,20 @@ static void __dma_rx_complete(void *param)
>  	tty_flip_buffer_push(tty_port);
>  }
>  
> +static void dma_rx_complete(void *param)
> +{
> +	struct uart_8250_port *p = param;
> +	struct uart_8250_dma *dma = p->dma;
> +	unsigned long flags;
> +
> +	__dma_rx_complete(p);
> +
> +	spin_lock_irqsave(&p->port.lock, flags);
> +	if (!dma->rx_running && (serial_lsr_in(p) & UART_LSR_DR))
> +		p->dma->rx_dma(p);
> +	spin_unlock_irqrestore(&p->port.lock, flags);
> +}
> +
>  int serial8250_tx_dma(struct uart_8250_port *p)
>  {
>  	struct uart_8250_dma		*dma = p->dma;
> @@ -130,7 +143,7 @@ int serial8250_rx_dma(struct uart_8250_port *p)
>  		return -EBUSY;
>  
>  	dma->rx_running = 1;
> -	desc->callback = __dma_rx_complete;
> +	desc->callback = dma_rx_complete;
>  	desc->callback_param = p;
>  
>  	dma->rx_cookie = dmaengine_submit(desc);
> -- 
> 2.30.2
> 

-- 
With Best Regards,
Andy Shevchenko