linux-kernel - Re: [PATCH] dmaengine: virt-dma: fix completion list manipulation

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Date:	Mon, 02 Mar 2015 23:03:33 +0100
From:	Robert Jarzmik <robert.jarzmik@...e.fr>
To:	Lars-Peter Clausen <lars@...afoo.de>
Cc:	Vinod Koul <vinod.koul@...el.com>, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dmaengine: virt-dma: fix completion list manipulation

Lars-Peter Clausen <lars@...afoo.de> writes:

> On 03/02/2015 10:19 PM, Robert Jarzmik wrote:
>> diff --git a/drivers/dma/virt-dma.h b/drivers/dma/virt-dma.h
>> index 3772032..2a3da22 100644
>> --- a/drivers/dma/virt-dma.h
>> +++ b/drivers/dma/virt-dma.h
>> @@ -91,7 +91,7 @@ static inline void vchan_cookie_complete(struct virt_dma_desc *vd)
>>   	dma_cookie_complete(&vd->tx);
>>   	dev_vdbg(vc->chan.device->dev, "txd %p[%x]: marked complete\n",
>>   		 vd, cookie);
>> -	list_add_tail(&vd->node, &vc->desc_completed);
>> +	list_move_tail(&vd->node, &vc->desc_completed);
>
> That will break all drivers which handle this currently correctly and remove the
> descriptor from any list before calling vchan_cookie_complete.
Ah, well well I don't agree.

First, let's split the drivers which remove the descriptors and these which
don't :

These which remove the descriptor:
dma-jz4740.c
fsl-edma.c

These which don't remove the descriptor:
amba-pl08x.c
edma.c
img-mdc-dma.c
k3dma.c
moxart-dma.c
omap-dma.c
qcom_bam_dma.c
s3c24xx-dma.c
sa11x0-dma.c
sun6i-dma.c

That settles the correctness I think, the correct behavior is to not remove the
descriptor and let it be done by vchan_cookie_complete().

Now for the remaining 2 drivers, we'll have :
 - list_del(&vd->node) => vd becomes a singleton
 - list_move_tail(&vd->node, &...desc_completed)
   => list_del(&vd->node) : nothing changes, it's a nop
   => list_add_tail(&vd->node, &...desc_completed)

And the behavior remains correct after the patch, only one "list_del()" is done
twice for nothing. Where do you see any breakage ?

Cheers.

-- 
Robert
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