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

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Date:	Mon, 18 Apr 2016 10:15:56 -0500
From:	Timur Tabi <timur@...eaurora.org>
To:	Sinan Kaya <okaya@...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

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.

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