linux-kernel - Re: [PATCH 3/3] dmaengine: at_xdmac: double FIFO flush needed to compute residue

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Date:	Fri, 13 May 2016 14:28:22 +0200
From:	Nicolas Ferre <nicolas.ferre@...el.com>
To:	Ludovic Desroches <ludovic.desroches@...el.com>,
	<vinod.koul@...el.com>
CC:	<linux-arm-kernel@...ts.infradead.org>,
	<dmaengine@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] dmaengine: at_xdmac: double FIFO flush needed to
 compute residue

Le 12/05/2016 16:54, Ludovic Desroches a écrit :
> Due to the way CUBC register is updated, a double flush is needed to
> compute an accurate residue. First flush aim is to get data from the DMA
> FIFO and second one ensures that we won't report data which are not in
> memory.
> 
> Signed-off-by: Ludovic Desroches <ludovic.desroches@...el.com>
> Fixes: e1f7c9eee707 ("dmaengine: at_xdmac: creation of the atmel
> eXtended DMA Controller driver")
> Cc: stable@...r.kernel.org #v4.1 and later

Reviewed-by: Nicolas Ferre <nicolas.ferre@...el.com>

> ---
>  drivers/dma/at_xdmac.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/at_xdmac.c b/drivers/dma/at_xdmac.c
> index b02494e..75bd662 100644
> --- a/drivers/dma/at_xdmac.c
> +++ b/drivers/dma/at_xdmac.c
> @@ -1425,7 +1425,16 @@ at_xdmac_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
>  	residue = desc->xfer_size;
>  	/*
>  	 * Flush FIFO: only relevant when the transfer is source peripheral
> -	 * synchronized.
> +	 * synchronized. Flush is needed before reading CUBC because data in
> +	 * the FIFO are not reported by CUBC. Reporting a residue of the
> +	 * transfer length while we have data in FIFO can cause issue.
> +	 * Usecase: atmel USART has a timeout which means I have received
> +	 * characters but there is no more character received for a while. On
> +	 * timeout, it requests the residue. If the data are in the DMA FIFO,
> +	 * we will return a residue of the transfer length. It means no data
> +	 * received. If an application is waiting for these data, it will hang
> +	 * since we won't have another USART timeout without receiving new
> +	 * data.
>  	 */
>  	mask = AT_XDMAC_CC_TYPE | AT_XDMAC_CC_DSYNC;
>  	value = AT_XDMAC_CC_TYPE_PER_TRAN | AT_XDMAC_CC_DSYNC_PER2MEM;
> @@ -1481,6 +1490,19 @@ at_xdmac_tx_status(struct dma_chan *chan, dma_cookie_t cookie,
>  	}
>  
>  	/*
> +	 * Flush FIFO: only relevant when the transfer is source peripheral
> +	 * synchronized. Another flush is needed here because CUBC is updated
> +	 * when the controller sends the data write command. It can lead to
> +	 * report data that are not written in the memory or the device. The
> +	 * FIFO flush ensures that data are really written.
> +	 */
> +	if ((desc->lld.mbr_cfg & mask) == value) {
> +		at_xdmac_write(atxdmac, AT_XDMAC_GSWF, atchan->mask);
> +		while (!(at_xdmac_chan_read(atchan, AT_XDMAC_CIS) & AT_XDMAC_CIS_FIS))
> +			cpu_relax();
> +	}
> +
> +	/*
>  	 * Remove size of all microblocks already transferred and the current
>  	 * one. Then add the remaining size to transfer of the current
>  	 * microblock.
> 


-- 
Nicolas Ferre