Date: Mon, 21 Mar 2016 16:04:43 +0100
From: Boris Brezillon <boris.brezillon@free-electrons.com>
To: Franklin S Cooper Jr <fcooper@ti.com>
Cc: nsekhar@ti.com, dwmw2@infradead.org, computersforpeace@gmail.com,
        rogerq@ti.com, tony@atomide.com, devicetree@vger.kernel.org,
        linux-omap@vger.kernel.org, linux-mtd@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 6/7] mtd: nand: omap2: Fix high memory dma prefetch
 transfer
Message-ID: <20160321160443.0a4165d1@bbrezillon>
In-Reply-To: <1457654203-20856-7-git-send-email-fcooper@ti.com>
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	<1457654203-20856-7-git-send-email-fcooper@ti.com>
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Hi Franklin,

On Thu, 10 Mar 2016 17:56:42 -0600
Franklin S Cooper Jr <fcooper@ti.com> wrote:

> Based on DMA documentation and testing using high memory buffer when
> doing dma transfers can lead to various issues including kernel
> panics.

I guess it all comes from the vmalloced buffer case, which are not
guaranteed to be physically contiguous (one of the DMA requirement,
unless you have an iommu).

> 
> To workaround this simply use cpu copy. The amount of high memory
> buffers used are very uncommon so no noticeable performance hit should
> be seen.

Hm, that's not necessarily true. UBI and UBIFS allocate their buffers
using vmalloc (vmalloced buffers fall in the high_memory region), and
those are likely to be dis-contiguous if you have NANDs with pages > 4k.

I recently posted patches to ease sg_table creation from any kind of
virtual address [1][2]. Can you try them and let me know if it fixes
your problem?

Thanks,

Boris

[1]https://lkml.org/lkml/2016/3/8/276
[2]https://lkml.org/lkml/2016/3/8/277


-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com