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

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Message-ID: <20220521104430.1212bed5@kernel.org>
Date:   Sat, 21 May 2022 10:44:30 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Måns Rullgård <mans@...sr.com>,
        Pantelis Antoniou <pantelis.antoniou@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        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

On Sat, 21 May 2022 06:44:41 +0000 Christophe Leroy wrote:
> > Hm, I think the patch is necessary, sorry if you're also saying that
> > and I'm misinterpreting.  
> 
> Well, I say the contrary.
> 
> On the mainline the patch may be applied as is, it won't harm.
> 
> However, it is gets applied to kernel 4.9 (based on the fixes: tag), it 
> will break the driver for at least powerpc 8xx.

I see, we should make a note of that in the commit message so it doesn't
get sucked into stable.

> > Without the dma_sync_single_for_cpu() if swiotlb is used the data
> > will not be copied back into the original buffer if there is no sync.  
> 
> I don't know how SWIOTLB works or even what it is, does any of the 
> microcontrollers embedding freescale ethernet uses that at all ?

AFAIU SWIOTLB basically forces the use of bounce buffers even if the
device can reach the entire DRAM. I think some people also use it for
added security? IDK. I mostly use it to check if I'm using the DMA API
"right" :)