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

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Date:   Mon, 28 Jun 2021 19:07:10 +0530
From:   "M Tamseel Shams" <m.shams@...sung.com>
To:     "'Krzysztof Kozlowski'" <krzysztof.kozlowski@...onical.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

> > 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

Hi Krzysztof,
Will fix the alignments in the next version.

Thanks & Regards,
Tamseel Shams