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

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Message-ID: <5714FB9D.4000405@codeaurora.org>
Date:	Mon, 18 Apr 2016 11:22:05 -0400
From:	Sinan Kaya <okaya@...eaurora.org>
To:	Timur Tabi <timur@...eaurora.org>,
	Christoph Hellwig <hch@...radead.org>
Cc:	linux-rdma@...r.kernel.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 4/18/2016 11:15 AM, Timur Tabi wrote:
> Sinan Kaya wrote:
>>
>> VMAP allows you to make several pages look contiguous to the CPU.
>> It can only be used against logical addresses returned from kmalloc
>> or alloc_page.
>>
>> You cannot take several virtually mapped addresses returned by dma_alloc_coherent
>> and try to make them virtually contiguous again.
>>
>> The code happens to work on other architectures by pure luck. AFAIK, dma_alloc_coherent
>> returns logical addresses on Intel systems until it runs out of DMA memory. After
>> that intel arch will also start returning virtually mapped addresses and this code
>> will also fail. ARM64 on the other hand always returns a virtually mapped address.
>>
>> The goal of this code is to allocate a bunch of page sized memory and make it look
>> contiguous. It is just using the wrong API. The correct API is either kmalloc or
>> alloc_page map it with dma_map_page not dma_alloc_coherent.
>>
>> The proper usage of dma_map_page requires code to call dma_sync API in correct
>> places to be compatible with noncoherent systems. This code is already assuming
>> coherency. It would be a nice to have dma_sync APIs in right places. There is no
>> harm in calling dma_sync API for coherent systems as they are no-ops in DMA mapping
>> layer whereas it is a cache flush for non-coherent systems.
> 
> The text would be a great addition to the patch description.
> 

I can do that on the next version.

-- 
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