netdev - Re: Regarding tx-nocache-copy in the Sheevaplug

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Message-ID: <20141016173401.GA16515@f1.synalogic.ca>
Date:	Thu, 16 Oct 2014 10:34:01 -0700
From:	Benjamin Poirier <benjamin.poirier@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Lluís Batlle i Rossell <viric@...ic.name>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Carles Pagès <page@...ata.homelinux.net>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Regarding tx-nocache-copy in the Sheevaplug

On 2014/10/15 15:45, Eric Dumazet wrote:
> On Wed, 2014-10-15 at 14:57 -0700, Benjamin Poirier wrote:
> > On 2014/10/13 12:52, Lluís Batlle i Rossell wrote:
> > > Hello,
> > > 
> > > on the 7th of January 2014 ths patch was applied:
> > > https://lkml.org/lkml/2014/1/7/307
> > > 
> > > [PATCH v2] net: Do not enable tx-nocache-copy by default
> > >         
> > > In the Sheevaplug (ARM Feroceon 88FR131 from Marvell) this made packets to be
> > > sent corrupted. I think this machine has something special about the cache.
> > > 
> > > Enabling back this tx-nocache-copy (as it used to be before the patch) the
> > > transfers work fine again. I think that most people, encountering this problem,
> > > completely disable the tx offload instead of enabling back this setting.
> > > 
> > > Is this an ARM kernel problem regarding this platform?
> > 
> > This is odd, only x86 defines ARCH_HAS_NOCACHE_UACCESS. On arm,
> > skb_do_copy_data_nocache() should end up using __copy_from_user()
> > regardless of tx-nocache-copy.
> 
>  kmap_atomic()/kunmap_atomic() is missing, so we lack
> __cpuc_flush_dcache_area() operations.
> 

You lost me there.
1) I don't see the link
2) It seems kmap_atomic and so on are there:
$ grep kmap_atomic System.map-3.16-2-kirkwood
c0014838 T kmap_atomic
c001491c T kmap_atomic_pfn
c00149a4 T kmap_atomic_to_page

MACH_KIRKWOOD selects CPU_FEROCEON which has
__cpuc_flush_dcache_area ->
	cpu_cache.flush_kern_dcache_area ->
		feroceon_flush_kern_dcache_area
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