netdev - Re: [PATCH v3 5/5] net: ep93xx_eth: fix DMA API violations

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Message-Id: <20110611.162600.1058381951078443013.davem@davemloft.net>
Date:	Sat, 11 Jun 2011 16:26:00 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mika.westerberg@....fi
Cc:	netdev@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	hsweeten@...ionengravers.com, rmallon@...il.com, ynezz@...e.cz
Subject: Re: [PATCH v3 5/5] net: ep93xx_eth: fix DMA API violations

From: Mika Westerberg <mika.westerberg@....fi>
Date: Sat, 11 Jun 2011 21:39:58 +0300

> Russell King said:
>>
>> So, to summarize what its doing:
>>
>> 1. It allocates buffers for rx and tx.
>> 2. It maps them with dma_map_single().
>>       This transfers ownership of the buffer to the DMA device.
>> 3. In ep93xx_xmit,
>> 3a. It copies the data into the buffer with skb_copy_and_csum_dev()
>>       This violates the DMA buffer ownership rules - the CPU should
>>       not be writing to this buffer while it is (in principle) owned
>>       by the DMA device.
>> 3b. It then calls dma_sync_single_for_cpu() for the buffer.
>>       This transfers ownership of the buffer to the CPU, which surely
>>       is the wrong direction.
>> 4. In ep93xx_rx,
>> 4a. It calls dma_sync_single_for_cpu() for the buffer.
>>       This at least transfers the DMA buffer ownership to the CPU
>>       before the CPU reads the buffer
>> 4b. It then uses skb_copy_to_linear_data() to copy the data out.
>>       At no point does it transfer ownership back to the DMA device.
>> 5. When the driver is removed, it dma_unmap_single()'s the buffer.
>>       This transfers ownership of the buffer to the CPU.
>> 6. It frees the buffer.
>>
>> While it may work on ep93xx, it's not respecting the DMA API rules,
>> and with DMA debugging enabled it will probably encounter quite a few
>> warnings.
> 
> This patch fixes these violations.
> 
> Signed-off-by: Mika Westerberg <mika.westerberg@....fi>
> Acked-by: Russell King <rmk+kernel@....linux.org.uk>
> Acked-by: H Hartley Sweeten <hsweeten@...ionengravers.com>
> Tested-by: Petr Stetiar <ynezz@...e.cz>

Applied.
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