linux-kernel - Re: [PATCH 3/4] serial: imx: adapt rx buffer and dma periods

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Date:   Mon, 16 Sep 2019 16:24:17 +0200
From:   Lucas Stach <l.stach@...gutronix.de>
To:     Philipp Puschmann <philipp.puschmann@...ix.com>,
        linux-kernel@...r.kernel.org
Cc:     linux-serial@...r.kernel.org, shawnguo@...nel.org,
        s.hauer@...gutronix.de, jslaby@...e.com, vkoul@...nel.org,
        linux-imx@....com, kernel@...gutronix.de,
        gregkh@...uxfoundation.org, dmaengine@...r.kernel.org,
        dan.j.williams@...el.com, festevam@...il.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/4] serial: imx: adapt rx buffer and dma periods

On Mi, 2019-09-11 at 16:49 +0200, Philipp Puschmann wrote:
> Using only 4 DMA periods for UART RX is very few if we have a high
> frequency of small transfers - like in our case using Bluetooth with many
> small packets via UART - causing many dma transfers but in each only
> filling a fraction of a single buffer. Such a case may lead to the
> situation that DMA RX transfer is triggered but no buffer is available.
> While we have addressed the dma handling already we still want to avoid
> UART RX FIFO overrun. So we decrease the size of the buffers and increase
> their number and the total buffer size.
> 
> Signed-off-by: Philipp Puschmann <philipp.puschmann@...ix.com>

Reasoning seems sound:

Reviewed-by: Lucas Stach <l.stach@...gutronix.de>

> ---
>  drivers/tty/serial/imx.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> index 57d6e6ba556e..cdc51569237c 100644
> --- a/drivers/tty/serial/imx.c
> +++ b/drivers/tty/serial/imx.c
> @@ -1028,8 +1028,6 @@ static void imx_uart_timeout(struct timer_list *t)
>  	}
>  }
>  
> -#define RX_BUF_SIZE	(PAGE_SIZE)
> -
>  /*
>   * There are two kinds of RX DMA interrupts(such as in the MX6Q):
>   *   [1] the RX DMA buffer is full.
> @@ -1112,7 +1110,8 @@ static void imx_uart_dma_rx_callback(void *data)
>  }
>  
>  /* RX DMA buffer periods */
> -#define RX_DMA_PERIODS 4
> +#define RX_DMA_PERIODS	16
> +#define RX_BUF_SIZE	(PAGE_SIZE / 4)
>  
>  static int imx_uart_start_rx_dma(struct imx_port *sport)
>  {