2016-03-31 06:40:31

by Jisheng Zhang

Subject: [PATCH] net: mvneta: remove useless RX descriptor prefetch

The rx descriptors are allocated using dma_alloc_coherent, so prefetch
doesn't really happen at all.

Signed-off-by: Jisheng Zhang <[email protected]>
---
drivers/net/ethernet/marvell/mvneta.c | 1 -
1 file changed, 1 deletion(-)

diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
index 5880871..6c09a27 100644
--- a/drivers/net/ethernet/marvell/mvneta.c
+++ b/drivers/net/ethernet/marvell/mvneta.c
@@ -757,7 +757,6 @@ mvneta_rxq_next_desc_get(struct mvneta_rx_queue *rxq)
int rx_desc = rxq->next_desc_to_proc;

rxq->next_desc_to_proc = MVNETA_QUEUE_NEXT_DESC(rxq, rx_desc);
- prefetch(rxq->descs + rxq->next_desc_to_proc);
return rxq->descs + rx_desc;
}

--
2.8.0.rc3

2016-03-31 06:49:55

by Jisheng Zhang

Subject: Re: [PATCH] net: mvneta: remove useless RX descriptor prefetch

Hi,

+ linux arm kernel

On Thu, 31 Mar 2016 14:36:30 +0800 Jisheng Zhang wrote:

> The rx descriptors are allocated using dma_alloc_coherent, so prefetch
> doesn't really happen at all.

This is for RFC, I'm sorry to send it without changing its title -- s/PATCH/RFC.

I'm not sure whether there's any benefit to prefetch on space allocated from
dma_alloc_coherent.

Thanks

>
> Signed-off-by: Jisheng Zhang <[email protected]>
> ---
> drivers/net/ethernet/marvell/mvneta.c | 1 -
> 1 file changed, 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> index 5880871..6c09a27 100644
> --- a/drivers/net/ethernet/marvell/mvneta.c
> +++ b/drivers/net/ethernet/marvell/mvneta.c
> @@ -757,7 +757,6 @@ mvneta_rxq_next_desc_get(struct mvneta_rx_queue *rxq)
> int rx_desc = rxq->next_desc_to_proc;
>
> rxq->next_desc_to_proc = MVNETA_QUEUE_NEXT_DESC(rxq, rx_desc);
> - prefetch(rxq->descs + rxq->next_desc_to_proc);
> return rxq->descs + rx_desc;
> }
>

2016-03-31 07:19:20

by Jisheng Zhang

Subject: Re: [PATCH] net: mvneta: remove useless RX descriptor prefetch

Hi all,

On Thu, 31 Mar 2016 14:45:37 +0800 Jisheng Zhang wrote:

> Hi,
>
> + linux arm kernel
>
> On Thu, 31 Mar 2016 14:36:30 +0800 Jisheng Zhang wrote:
>
> > The rx descriptors are allocated using dma_alloc_coherent, so prefetch
> > doesn't really happen at all.
>
> This is for RFC, I'm sorry to send it without changing its title -- s/PATCH/RFC.
>
> I'm not sure whether there's any benefit to prefetch on space allocated from
> dma_alloc_coherent.

After more consideration, I think my patch is wrong.

As for coherent platforms, the space allocated from dma_alloc_coherent is
cacheable, so prefetch would definitely benefit us.

As for noncoherent platforms, the space allocated from dma_alloc_coherent is
uncacheable, but prefetch on arm/arm64 is implemented via. pld/prfm, the op
would be nop if target address is uncacheable.

So let's drop this patch.

Thanks,
Jisheng


>
> Thanks
>
> >
> > Signed-off-by: Jisheng Zhang <[email protected]>
> > ---
> > drivers/net/ethernet/marvell/mvneta.c | 1 -
> > 1 file changed, 1 deletion(-)
> >
> > diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> > index 5880871..6c09a27 100644
> > --- a/drivers/net/ethernet/marvell/mvneta.c
> > +++ b/drivers/net/ethernet/marvell/mvneta.c
> > @@ -757,7 +757,6 @@ mvneta_rxq_next_desc_get(struct mvneta_rx_queue *rxq)
> > int rx_desc = rxq->next_desc_to_proc;
> >
> > rxq->next_desc_to_proc = MVNETA_QUEUE_NEXT_DESC(rxq, rx_desc);
> > - prefetch(rxq->descs + rxq->next_desc_to_proc);
> > return rxq->descs + rx_desc;
> > }
> >
>
>
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