linux-kernel - Re: [PATCH v4] serial: samsung: use dma

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Date:   Mon, 28 Jun 2021 10:44:51 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Tamseel Shams <m.shams@...sung.com>, gregkh@...uxfoundation.org,
        jirislaby@...nel.org
Cc:     linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, alim.akhtar@...sung.com,
        ajaykumar.rs@...sung.com, robin.murphy@....com
Subject: Re: [PATCH v4] serial: samsung: use dma_ops of DMA if attached

On 25/06/2021 09:51, Tamseel Shams wrote:
> When DMA is used for TX and RX by serial driver, it should
> pass the DMA device pointer to DMA API instead of UART device
> pointer. DMA device should be used for DMA API because only
> the DMA device is aware of how the device connects to the memory.
> There might be an extra level of address translation due to a
> SMMU attached to the DMA device. When serial device is used for
> DMA API, the DMA API will have no clue of the SMMU attached to
> the DMA device.
> 
> This patch is necessary to fix the SMMU page faults
> which is observed when a DMA(with SMMU enabled) is attached
> to UART for transfer.
> 
> Signed-off-by: Tamseel Shams <m.shams@...sung.com>
> Signed-off-by: Ajay Kumar <ajaykumar.rs@...sung.com>
> ---
> Changes since v1:
> 1. Rebased the patch on "tty-next" branch of TTY driver tree
> 
> Changes since v2:
> 1. Updated the commit message.
> 2. Changed the comment description
> 
> Changes since v3:
> 1. Removed the null pointer check for "dma", "dma->tx_chan" and
> "dma->rx_chan" and instead sending DMA device pointer while calling
> DMA API.
> 
>  drivers/tty/serial/samsung_tty.c | 32 ++++++++++++++++----------------
>  1 file changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
> index 9fbc61151c2e..fa30aa20a13f 100644
> --- a/drivers/tty/serial/samsung_tty.c
> +++ b/drivers/tty/serial/samsung_tty.c
> @@ -305,7 +305,7 @@ static void s3c24xx_serial_stop_tx(struct uart_port *port)
>  		dmaengine_pause(dma->tx_chan);
>  		dmaengine_tx_status(dma->tx_chan, dma->tx_cookie, &state);
>  		dmaengine_terminate_all(dma->tx_chan);
> -		dma_sync_single_for_cpu(ourport->port.dev,
> +		dma_sync_single_for_cpu(dma->tx_chan->device->dev,
>  			dma->tx_transfer_addr, dma->tx_size, DMA_TO_DEVICE);
>  		async_tx_ack(dma->tx_desc);
>  		count = dma->tx_bytes_requested - state.residue;
> @@ -338,8 +338,8 @@ static void s3c24xx_serial_tx_dma_complete(void *args)
>  	count = dma->tx_bytes_requested - state.residue;
>  	async_tx_ack(dma->tx_desc);
>  
> -	dma_sync_single_for_cpu(ourport->port.dev, dma->tx_transfer_addr,
> -				dma->tx_size, DMA_TO_DEVICE);
> +	dma_sync_single_for_cpu(dma->tx_chan->device->dev,
> +			dma->tx_transfer_addr, dma->tx_size, DMA_TO_DEVICE);

Looks like alignment is broken here. However even if the line was not
aligned before, please fix it up now - align the arguments like
checkpatch suggests.

This applies to other places as well. Thanks.

Best regards,
Krzysztof