netdev - Re: [PATCH net v6] sfc: PIO:Restrict to 64bit arch and use 64-bit writes.

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Date:	Wed, 11 Jun 2014 15:36:53 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	sshah@...arflare.com
Cc:	netdev@...r.kernel.org, linux-net-drivers@...arflare.com
Subject: Re: [PATCH net v6] sfc: PIO:Restrict to 64bit arch and use 64-bit
 writes.

From: Shradha Shah <sshah@...arflare.com>
Date: Wed, 11 Jun 2014 14:33:08 +0100

> From: Jon Cooper <jcooper@...arflare.com>
> 
> Fixes:ee45fd92c739
> ("sfc: Use TX PIO for sufficiently small packets")
> 
> The linux net driver uses memcpy_toio() in order to copy into
> the PIO buffers.
> Even on a 64bit machine this causes 32bit accesses to a write-
> combined memory region.
> There are hardware limitations that mean that only 64bit
> naturally aligned accesses are safe in all cases.
> Due to being write-combined memory region two 32bit accesses
> may be coalesced to form a 64bit non 64bit aligned access.
> Solution was to open-code the memory copy routines using pointers
> and to only enable PIO for x86_64 machines.
> 
> Not tested on platforms other than x86_64 because this patch
> disables the PIO feature on other platforms.
> Compile-tested on x86 to ensure that works.
> 
> The WARN_ON_ONCE() code in the previous version of this patch
> has been moved into the internal sfc debug driver as the
> assertion was unnecessary in the upstream kernel code.
> 
> This bug fix applies to v3.13 and v3.14 stable branches.
> 
> Signed-off-by: Shradha Shah <sshah@...arflare.com>

Applied and queued up for -stable, thanks.
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