linux-kernel - Re: [PATCH V2] net: ethernet: mellanox: correct page conversion

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Date:	Mon, 18 Apr 2016 11:59:02 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	okaya@...eaurora.org
Cc:	linux-rdma@...r.kernel.org, timur@...eaurora.org,
	cov@...eaurora.org, yishaih@...lanox.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] net: ethernet: mellanox: correct page conversion

From: okaya@...eaurora.org
Date: Mon, 18 Apr 2016 01:06:27 -0400

> On 2016-04-18 00:00, David Miller wrote:
>> From: Sinan Kaya <okaya@...eaurora.org>
>> Date: Sat, 16 Apr 2016 18:23:32 -0400
>> 
>>> Current code is assuming that the address returned by
>>> dma_alloc_coherent
>>> is a logical address. This is not true on ARM/ARM64 systems. This
>>> patch
>>> replaces dma_alloc_coherent with dma_map_page API. The address
>>> returned
>>> can later by virtually mapped from the CPU side with vmap API.
>>> Signed-off-by: Sinan Kaya <okaya@...eaurora.org>
>> You can't do this.
>> The DMA map page API gives non-coherent mappings, and thus requires
>> proper flushing.
>> So a straight conversion like this is never legitimate.
> 
> I would agree on proper dma api usage. However, the code is already
> assuming coherent architecture by mapping the cpu pages as
> page_kernel.
> 
> Dma_map_page returns cached buffers and you don't need cache flushes
> on coherent architecture to make the data visible.

All you are telling me is that there are two bugs instead of one, so now
both need to be fixed.