linux-kernel - Re: [PATCH v4 6/7] mtd: nand: omap2: Fix high memory dma prefetch transfer

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Date:	Mon, 21 Mar 2016 16:04:43 +0100
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Franklin S Cooper Jr <fcooper@...com>
Cc:	nsekhar@...com, dwmw2@...radead.org, computersforpeace@...il.com,
	rogerq@...com, tony@...mide.com, devicetree@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-mtd@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/7] mtd: nand: omap2: Fix high memory dma prefetch
 transfer

Hi Franklin,

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