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

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Date:	Wed, 13 Apr 2016 15:08:12 -0500
From:	"Franklin S Cooper Jr." <fcooper@...com>
To:	Boris Brezillon <boris.brezillon@...e-electrons.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



On 03/21/2016 10:04 AM, Boris Brezillon wrote:
> 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?

It looks like you won't be going forward with your patchset based on
this thread [1]. I can probably reword the patch description to avoid
implying that it is uncommon to run into high mem buffers. Also DMA with
NAND prefetch suffers from a reduction of performance compared to CPU
polling with prefetch. This is largely due to the significant over head
required to read such a small amount of data at a time. The
optimizations I've worked on all revolved around reducing the cycles
spent before executing the DMA request. Trying to make a high memory
buffer able to be used by the DMA adds significant amount of cycles and
your better off just using the cpu for performance reasons.

[1]https://lkml.org/lkml/2016/4/4/346
> 
> Thanks,
> 
> Boris
> 
> [1]https://lkml.org/lkml/2016/3/8/276
> [2]https://lkml.org/lkml/2016/3/8/277
> 
>