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

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Message-ID: <bd6ff4fb-0cbd-675e-a4f2-d311cfe2c62d@emlix.com>
Date:   Thu, 19 Sep 2019 11:20:16 +0200
From:   Philipp Puschmann <philipp.puschmann@...ix.com>
To:     Lucas Stach <l.stach@...gutronix.de>, 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

Am 16.09.19 um 16:17 schrieb Lucas Stach:
> 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

Hi Lucas,

thanks for your feedback. I will apply the hints to v2 of the patches.

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