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

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Date:	Tue, 19 Apr 2016 14:37:42 -0400
From:	Sinan Kaya <okaya@...eaurora.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-rdma@...r.kernel.org, timur@...eaurora.org,
	cov@...eaurora.org, Yishai Hadas <yishaih@...lanox.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] net: ethernet: mellanox: correct page conversion

On 4/19/2016 2:22 PM, Christoph Hellwig wrote:
> What I think we need is something like the patch below.  In the long
> ru nwe should also kill the mlx4_buf structure which now is pretty
> pointless.

Maybe; this could be the correct approach if we can guarantee that the 
architecture can allocate the requested amount of memory with
dma_alloc_coherent. 

When I brought this issue a year ago, the objection was that my code
doesn't compile on intel (dma_to_phys) and also some arches run out of
DMA memory with existing customer base.

If there is a real need to maintain compatibility with the existing 
architectures due to limited amount of DMA memory, we need to correct this
code to make proper use of vmap via alloc_pages and also insert the 
dma_sync in proper places for DMA API conformance. 

Also, the tx descriptors always has to be allocated from a single DMA region
or the tx code needs to be corrected to support page_list.

If we can live with just using dma_alloc_coherent, your solution is
better. I was trying to put this support for 64bit arches only while
maintaining support for the existing code base.

> 
> ---
> From a493881d2a6c90152d3daabb7b6b3afd1d254d78 Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@....de>
> Date: Tue, 19 Apr 2016 14:12:14 -0400
> Subject: mlx4_en: don't try to split and vmap dma coherent allocations
> 
> The memory returned by dma_alloc_coherent is not suitable for calling
> virt_to_page on it, as it might for example come from vmap allocator.
> 
> Remove the code that calls virt_to_page and vmap on dma coherent
> allocations from the mlx4 drivers, and replace them with a single
> high-order dma_alloc_coherent call.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> Reported-by: Sinan Kaya <okaya@...eaurora.org>

-- 
Sinan Kaya
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project