linux-kernel - Re: [PATCH v2] dmaengine: xgene-dma: Disable memcpy operation due to performance drop

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Date:	Mon, 12 Oct 2015 19:53:36 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Rameshwar Sahu <rsahu@....com>
Cc:	dan.j.williams@...el.com, dmaengine@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	jcm@...hat.com, patches@....com
Subject: Re: [PATCH v2] dmaengine: xgene-dma: Disable memcpy operation due to
 performance drop

On Fri, Oct 09, 2015 at 10:08:32PM +0530, Rameshwar Sahu wrote:
> Hi Vinod,
> 
> On Fri, Oct 9, 2015 at 9:42 PM, Vinod Koul <vinod.koul@...el.com> wrote:
> > On Thu, Oct 08, 2015 at 02:36:57PM +0530, Rameshwar Prasad Sahu wrote:
> >> The DMA engine supports memory copy, RAID5 XOR, RAID6 PQ, and other
> >> computations. But the bandwidth of the entire DMA engine is shared
> >> among all channels. This patch re-configures operations availability
> >> such that one can achieve maximum performance for XOR and PQ
> >> computation by removing the memory offload operations.
> >>
> >> Signed-off-by: Rameshwar Prasad Sahu <rsahu@....com>
> >> ---
> >>  drivers/dma/xgene-dma.c | 1 -
> >>  1 file changed, 1 deletion(-)
> >>
> >> diff --git a/drivers/dma/xgene-dma.c b/drivers/dma/xgene-dma.c
> >> index 8d57b1b..cd38022 100644
> >> --- a/drivers/dma/xgene-dma.c
> >> +++ b/drivers/dma/xgene-dma.c
> >> @@ -1707,7 +1707,6 @@ static void xgene_dma_set_caps(struct xgene_dma_chan *chan,
> >>       dma_cap_zero(dma_dev->cap_mask);
> >>
> >>       /* Set DMA device capability */
> >> -     dma_cap_set(DMA_MEMCPY, dma_dev->cap_mask);
> >>       dma_cap_set(DMA_SG, dma_dev->cap_mask);
> >
> > This will not work. Still clients can invoke memcpy operation. You need to
> > remove the callback
> 
> Got it, instead of removing the callback registration can we keep it
> inside the condition like..
> 
> if (dma_has_cap(DMA_MEMCPY, dma_dev->cap_mask))
>             dma_dev->device_prep_dma_memcpy = xgene_dma_prep_memcpy;

But the code is dead and will not be tested and hence not maintained. I do
not see a reason to keep dead code in kernel

-- 
~Vinod
--
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