netdev - Re: [PATCH] net: fs_enet: sync rx dma buffer before reading

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Message-ID: <yw1xmtfc9yaj.fsf@mansr.com>
Date:   Fri, 20 May 2022 13:35:48 +0100
From:   Måns Rullgård <mans@...sr.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Pantelis Antoniou <pantelis.antoniou@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Vitaly Bordug <vbordug@...mvista.com>,
        Dan Malek <dan@...eddededge.com>,
        Joakim Tjernlund <joakim.tjernlund@...entis.se>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: fs_enet: sync rx dma buffer before reading

Christophe Leroy <christophe.leroy@...roup.eu> writes:

> Le 19/05/2022 à 21:24, Mans Rullgard a écrit :
>> The dma_sync_single_for_cpu() call must precede reading the received
>> data. Fix this.
>
> See original commit 070e1f01827c. It explicitely says that the cache 
> must be invalidate _AFTER_ the copy.
>
> The cache is initialy invalidated by dma_map_single(), so before the 
> copy the cache is already clean.
>
> After the copy, data is in the cache. In order to allow re-use of the 
> skb, it must be put back in the same condition as before, in extenso the 
> cache must be invalidated in order to be in the same situation as after 
> dma_map_single().
>
> So I think your change is wrong.

OK, looking at it more closely, the change is at least unnecessary since
there will be a cache invalidation between each use of the buffer either
way.  Please disregard the patch.  Sorry for the noise.

>> 
>> Fixes: 070e1f01827c ("net: fs_enet: don't unmap DMA when packet len is below copybreak")
>> Signed-off-by: Mans Rullgard <mans@...sr.com>
>> ---
>>   drivers/net/ethernet/freescale/fs_enet/fs_enet-main.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>> 
>> diff --git a/drivers/net/ethernet/freescale/fs_enet/fs_enet-main.c b/drivers/net/ethernet/freescale/fs_enet/fs_enet-main.c
>> index b3dae17e067e..432ce10cbfd0 100644
>> --- a/drivers/net/ethernet/freescale/fs_enet/fs_enet-main.c
>> +++ b/drivers/net/ethernet/freescale/fs_enet/fs_enet-main.c
>> @@ -240,14 +240,14 @@ static int fs_enet_napi(struct napi_struct *napi, int budget)
>>                                  /* +2 to make IP header L1 cache aligned */
>>                                  skbn = netdev_alloc_skb(dev, pkt_len + 2);
>>                                  if (skbn != NULL) {
>> +                                       dma_sync_single_for_cpu(fep->dev,
>> +                                               CBDR_BUFADDR(bdp),
>> +                                               L1_CACHE_ALIGN(pkt_len),
>> +                                               DMA_FROM_DEVICE);
>>                                          skb_reserve(skbn, 2);   /* align IP header */
>>                                          skb_copy_from_linear_data(skb,
>>                                                        skbn->data, pkt_len);
>>                                          swap(skb, skbn);
>> -                                       dma_sync_single_for_cpu(fep->dev,
>> -                                               CBDR_BUFADDR(bdp),
>> -                                               L1_CACHE_ALIGN(pkt_len),
>> -                                               DMA_FROM_DEVICE);
>>                                  }
>>                          } else {
>>                                  skbn = netdev_alloc_skb(dev, ENET_RX_FRSIZE);
>> --
>> 2.35.1
>> 

-- 
Måns Rullgård