netdev - RE: [PATCH v2 5/5] net: ep93xx_eth: fix DMA API violations

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Message-ID: <ADE657CA350FB648AAC2C43247A983F001F3820B8639@AUSP01VMBX24.collaborationhost.net>
Date:	Thu, 9 Jun 2011 16:25:02 -0500
From:	H Hartley Sweeten <hartleys@...ionengravers.com>
To:	Mika Westerberg <mika.westerberg@....fi>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"kernel@...tstofly.org" <kernel@...tstofly.org>,
	"ryan@...ewatersys.com" <ryan@...ewatersys.com>
Subject: RE: [PATCH v2 5/5] net: ep93xx_eth: fix DMA API violations

On Thursday, June 02, 2011 12:00 PM, Mika Westerberg wrote:
> 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>

Well... I'm not going to even pretend to actually understand the DMA API
at this point.  But, this patch seems to follow what DMA-API-HOWTO.txt
describes as the proper usage of the DMA API.

Also, with this patch and the others in your series my test systems are
still booting and working properly.

Acked-by: H Hartley Sweeten <hsweeten@...ionengravers.com>--
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