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

From: Benjamin Poirier <[email-masked]>
Date: Wed, 15 Oct 2014 14:57:01 -0700

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.