linux-kernel - Re: [PATCH 1/4] dmaengine: imx-sdma: fix buffer ownership

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Message-ID: <9bcf315369449a025828410396935b679aae14bf.camel@pengutronix.de>
Date:   Mon, 16 Sep 2019 16:17:00 +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 1/4] dmaengine: imx-sdma: fix buffer ownership

On Mi, 2019-09-11 at 16:49 +0200, Philipp Puschmann wrote:
> BD_DONE flag marks ownership of the buffer. When 1 SDMA owns the buffer,
> when 0 ARM owns it. When processing the buffers in
> sdma_update_channel_loop the ownership of the currently processed buffer
> was set to SDMA again before running the callback function of the the
> buffer and while the sdma script may be running in parallel. So there was
> the possibility to get the buffer overwritten by SDMA before it has been
> processed by kernel leading to kind of random errors in the upper layers,
> e.g. bluetooth.
> 
> It may be further a good idea to make the status struct member volatile or
> access it using writel or similar to rule out that the compiler sets the
> BD_DONE flag before the callback routine has finished.
> 
> Signed-off-by: Philipp Puschmann <philipp.puschmann@...ix.com>
> ---
>  drivers/dma/imx-sdma.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/dma/imx-sdma.c b/drivers/dma/imx-sdma.c
> index a01f4b5d793c..1abb14ff394d 100644
> --- a/drivers/dma/imx-sdma.c
> +++ b/drivers/dma/imx-sdma.c
> @@ -802,7 +802,6 @@ static void sdma_update_channel_loop(struct sdma_channel *sdmac)
>  		*/
>  
>  		desc->chn_real_count = bd->mode.count;
> -		bd->mode.status |= BD_DONE;
>  		bd->mode.count = desc->period_len;
>  		desc->buf_ptail = desc->buf_tail;
>  		desc->buf_tail = (desc->buf_tail + 1) % desc->num_bd;
> @@ -817,6 +816,8 @@ static void sdma_update_channel_loop(struct sdma_channel *sdmac)
>  		dmaengine_desc_get_callback_invoke(&desc->vd.tx, NULL);
>  		spin_lock(&sdmac->vc.lock);

To address your comment from the second paragraph of the commit message
there should be a dma_wmb() here before changing the status flag.

Regards,
Lucas

> +		bd->mode.status |= BD_DONE;
> +
>  		if (error)
>  			sdmac->status = old_status;
>  	}