linux-kernel - Re: build_skb() and data corruption

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Message-Id: <201401141651.34633.arnd@arndb.de>
Date:	Tue, 14 Jan 2014 16:51:34 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Jonas Jensen <jonas.jensen@...il.com>
Cc:	netdev <netdev@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	alexander.h.duyck@...el.com,
	Florian Fainelli <florian@...nwrt.org>,
	Ben Hutchings <bhutchings@...arflare.com>
Subject: Re: build_skb() and data corruption

On Tuesday 14 January 2014, Jonas Jensen wrote:
> Thanks for the replies, you led me to a new solution,
> 
> 
> I now think build_skb() is not the right choice, my motivation for
> using it in the first place, that I thought it meant getting away with
> not copying memory.
> 
> build_skb() is replaced by netdev_alloc_skb_ip_align() and memcpy()
> (derived from drivers/net/ethernet/realtek/r8169.c).
> 
> Read errors are gone, even without syncing DMA. Is it a good idea to
> do it anyway, i.e. leave calls to dma_sync_single_* in?

The calls to dma_sync_single_* in the moxart_rx() function are needed.
The call to arm_dma_ops.sync_single_for_device() in
moxart_mac_setup_desc_ring() is wrong, because the buffer is already
owned by the device at that point (just after dma_map_single), and
because you should use the official dma_* api rather than using
the arm_dma_ops struct.

	Arnd
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