netdev - Re: [PATCH net-next 1/5] nfp: make use of the DMA_ATTR_SKIP_CPU

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Date:   Fri, 21 Apr 2017 15:37:45 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     alexander.duyck@...il.com
Cc:     jakub.kicinski@...ronome.com, netdev@...r.kernel.org,
        oss-drivers@...ronome.com, kubakici@...pl
Subject: Re: [PATCH net-next 1/5] nfp: make use of the
 DMA_ATTR_SKIP_CPU_SYNC attr

From: Alexander Duyck <alexander.duyck@...il.com>
Date: Fri, 21 Apr 2017 08:07:45 -0700

> On Fri, Apr 21, 2017 at 7:20 AM, Jakub Kicinski
> <jakub.kicinski@...ronome.com> wrote:
>> DMA unmap may destroy changes CPU made to the buffer.  To make XDP
>> run correctly on non-x86 platforms we should use the
>> DMA_ATTR_SKIP_CPU_SYNC attribute.
>>
>> Thanks to using the attribute we can now push the sync operation to
>> the common code path from XDP handler.
>>
>> A little bit of variable name reshuffling is required to bring the
>> code back to readable state.
>>
>> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> 
> So I see where you added the sync_single_for_cpu, but what about the
> sync_single_for_device? It needs to be called for the buffer before
> you assign it for Rx. On x86 it won't really matter but for proper
> utilization of the DMA API you need to sync the buffer even for Rx
> just to make certain that the cache lines are evicted prior to the
> device attempting to write to the buffer.

Agreed, a sync before giving the buffer to the device is necessary.