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

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Date:	Thu, 16 Oct 2014 19:46:28 +0200
From:	Lluís Batlle i Rossell <viric@...ic.name>
To:	Benjamin Poirier <benjamin.poirier@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	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 Thu, Oct 16, 2014 at 10:34:01AM -0700, Benjamin Poirier wrote:
> 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

Hello all,

it seems I was a bit wrong - although enabling back tx-nocache-copy makes the
tx-errors happen much less often (ssh complaining about HMAC), they still
happen. It seems that something was introduced in some recent kernels that broke
the tx offload.

I have no idea what it can be, but since 2.6 until at least 3.10 the network
driver worked fine with tx offload in this sheevaplug board.

Regards,
Lluís.
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