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

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Message-ID: <b11dcb32-5915-c1c8-9f0e-3cfc57b55792@csgroup.eu>
Date:   Fri, 20 May 2022 12:54:56 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Måns Rullgård <mans@...sr.com>
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



Le 20/05/2022 à 14:35, Måns Rullgård a écrit :
> 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.
> 

I also looked deeper.

Indeed it was implemented in kernel 4.9 or 4.8. At that time 
dma_unmap_single() was a no-op, it was not doing any sync/invalidation 
at all, invalidation was done only at mapping, so when we were reusing 
the skb it was necessary to clean the cache _AFTER_ the copy as if it 
was a new mapping.

Today a sync is done at both map and unmap, so it doesn't really matter 
whether we do the invalidation before or after the copy when we re-use 
the skb.

Christophe