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

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Message-Id: <20160418.000039.1946438541182822213.davem@davemloft.net>
Date:	Mon, 18 Apr 2016 00:00:39 -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: 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.